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In Python 3.4.3, the doc string of the str() builtin still reads:
"str(object='') -> str\nstr(bytes_or_buffer[, encoding[, errors]]) -> str\n\nCreate a new string object from the given object. If encoding or\nerrors is specified, then the object must expose a data buffer\nthat will be decoded using the given encoding and error handler.\nOtherwise, returns the result of object.__str__() (if defined)\nor repr(object).\nencoding defaults to sys.getdefaultencoding().\nerrors defaults to 'strict'."
This no longer matches what str() does. sys.getdefaultencoding() always returns 'utf-8' and str() accepts bytes, bytearray or buffer-like objects as input when using an encoding, any object with .__str__() method defined or repr(obj) otherwise.

Slightly easier to read version:
>>> pprint(str.__doc__)
("str(object='') -> str\n"
 'str(bytes_or_buffer[, encoding[, errors]]) -> str\n'
 '\n'
 'Create a new string object from the given object. If encoding or\n'
 'errors is specified, then the object must expose a data buffer\n'
 'that will be decoded using the given encoding and error handler.\n'
 'Otherwise, returns the result of object.__str__() (if defined)\n'
 'or repr(object).\n'
 'encoding defaults to sys.getdefaultencoding().\n'
 "errors defaults to 'strict'.")
I don’t exactly understand your complaint.
Do you want to say encoding directly defaults to UTF-8, bypassing the getdefaultencoding() reference?
Do you want to explicitly point out that bytes() and bytearray() expose a data buffer and can therefore be decoded? This is already hinted in the name bytes_or_buffer.
Do you want to clarify that the Otherwise sentence applies when encoding and errors are not specified, rather than when the object does not expose a buffer?
Maybe another thing to change could be fixing the second signature, to show that an explicit encoding can be omitted, and bytes_or_buffer is not a valid keyword name:
str(object, encoding="utf-8", errors="strict")

The current docstring is correct. The signature str(object, encoding="utf-8", errors="strict") is not correct, because str(object) is not the same as str(object, encoding="utf-8", errors="strict").
>>> str([1, 2])
'[1, 2]'
>>> str([1, 2], encoding="utf-8", errors="strict")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: decoding to str: need a bytes-like object, list found
>>> str(b'abc')
__main__:1: BytesWarning: str() on a bytes instance
"b'abc'"
>>> str(b'abc', encoding="utf-8", errors="strict")
'abc'