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The proposed patch adds link to alternative interfaces for bytes-to-bytes codecs. I.e. base64.b64encode and base64.b64decode for base64_codec.
Patch for 2.7 should mention other functions/modules (due to lack of some of them).

Any opinions?

I like this, both because it quite clearly defines the encode and decode directions, and allows notes the more direct entry points if the codec isn't being specified as an input string.
So +1 from me.

Not a bad idea. More information is always better when it comes to
documentation :-)

> Not a bad idea.
How about implementation? Here is updated patches for 3.x and 2.7. Note that in 2.7 I split codecs table as in 3.x.

I like the idea of splitting the table in 2.7 rather than using a result type column. However, the two intro paragraphs need a bit of work. How does the following sound:
1. Create a new subheading at the same level as the current "Standard Encodings" heading: "Python Specific Encodings"
2. Split out rot-13 to its own table in Python 2.7 as well
3. Under the new subheading, have the following text introducing the tables:
----
A number of predefined codecs are specific to Python, so their codec names have no meaning outside Python. These are listed in the tables below based on the expected input and output types (note that while text encodings are the most common use case for codecs, the underlying codec infrastructure supports arbitrary data transforms rather than just text encodings). For asymmetric codecs, the stated purpose describes the encoding direction.
The following codecs provide text-to-binary encoding and binary-to-text decoding, similar to the Unicode text encodings.
----
The following codecs provide binary-to-binary encoding and decoding.
----
The following codecs provide text-to-text encoding and decoding.
----

> However, the two intro paragraphs need a bit of work.
Yes, it's a help which I needed. Thank you.
However your wording is not entirely correct. In 2.7 binary-to-binary codecs and rot-13 works with Unicode strings (only ascii-compatible) as with bytes strings.
>>> u'Python'.encode('base64')
'UHl0aG9u\n'
>>> u'UHl0aG9u'.decode('base64')
'Python'
>>> u'Python\u20ac'.encode('base64')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/serhiy/py/cpython-2.7/Lib/encodings/base64_codec.py", line 24, in base64_encode
 output = base64.encodestring(input)
 File "/home/serhiy/py/cpython-2.7/Lib/base64.py", line 315, in encodestring
 pieces.append(binascii.b2a_base64(chunk))
UnicodeEncodeError: 'ascii' codec can't encode character u'\u20ac' in position 6: ordinal not in range(128)
Rot-13 works as common text-to-binary encoding (encode returns str, decode returns unicode).
>>> u'Python'.encode('rot13')
'Clguba'
>>> u'Python'.decode('rot13')
u'Clguba'
>>> 'Python'.encode('rot13')
'Clguba'
>>> 'Python'.decode('rot13')
u'Clguba'
>>> u'Python\u20ac'.encode('rot13')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/serhiy/py/cpython-2.7/Lib/encodings/rot_13.py", line 17, in encode
 return codecs.charmap_encode(input,errors,encoding_map)
UnicodeEncodeError: 'charmap' codec can't encode character u'\u20ac' in position 6: character maps to <undefined>
>>> u'Python\u20ac'.decode('rot13')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/serhiy/py/cpython-2.7/Lib/encodings/rot_13.py", line 20, in decode
 return codecs.charmap_decode(input,errors,decoding_map)
UnicodeEncodeError: 'ascii' codec can't encode character u'\u20ac' in position 6: ordinal not in range(128)

While the Python 2 text model was almost certainly a necessary transition step to full unicode support, it is things like this that highlight how fundamentally broken implicit conversion turned out to be at a conceptual level :P
Perhaps the following would work for 2.7 then (with rot-13 in the first table), with footnotes added to cover the quirks of the implicit type conversions between str and unicode:
----
A number of predefined codecs are specific to Python, so their codec names have no meaning outside Python. These are listed in the tables below based on the expected input and output types (note that while text encodings are the most common use case for codecs, the underlying codec infrastructure supports arbitrary data transforms rather than just text encodings). For asymmetric codecs, the stated purpose describes the encoding direction.
The following codecs provide unicode-to-str encoding [#1] and str-to-unicode decoding [#2], similar to the Unicode text encodings.
----
The following codecs provide str-to-str encoding and decoding [#2].
----
.. [#1] str objects are also accepted as input in place of unicode objects. They are implicitly converted to unicode by decoding them using the default encoding. If this conversion fails, it may lead to encoding operations raising :exc:`UnicodeDecodeError`.
.. [#2] unicode objects are also accepted as input in place of str objects. They are implicitly converted to str by encoding them using the default encoding. If this conversion fails, it may lead to decoding operations raising :exc:`UnicodeEncodeError`.

Thank you Nick. Here is an updated patch for 2.7.

Thanks Serhiy, that version looks great.

New changeset 85c04fdaa404 by Serhiy Storchaka in branch '2.7':
Issue #17844: Refactor a documentation of Python specific encodings.
http://hg.python.org/cpython/rev/85c04fdaa404
New changeset 039dc6dd2bc0 by Serhiy Storchaka in branch '3.3':
Issue #17844: Add links to encoders and decoders for bytes-to-bytes codecs.
http://hg.python.org/cpython/rev/039dc6dd2bc0
New changeset 9afdd88fe33a by Serhiy Storchaka in branch 'default':
Issue #17844: Add links to encoders and decoders for bytes-to-bytes codecs.
http://hg.python.org/cpython/rev/9afdd88fe33a 

Thank you Nick. It's mainly your patch.
Do you want to foreport your changes (a "Python Specific Encodings" subheading and followed paragraph) to 3.x?

That sounds like a good idea. Yay for not needing those arcane footnotes, though :)

New changeset 85e8414060b4 by Nick Coghlan in branch '3.3':
Issue 17844: Clarify meaning of different codec tables
http://hg.python.org/cpython/rev/85e8414060b4
New changeset 801567d6302c by Nick Coghlan in branch 'default':
Merge issue 17844 from 3.3
http://hg.python.org/cpython/rev/801567d6302c 

Thanks for initiating this Serhiy :)