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These methods are parallel to str.join, seem to work as expected, and
have "help" entries. However there is nothing in the Library Reference
Manual about them.
>>> help(bytearray.join)
Help on method_descriptor:
join(...)
 B.join(iterable_of_bytes) -> bytearray
 Concatenate any number of bytes/bytearray objects, with B
 in between each pair, and return the result as a new bytearray.
### OK but could use an example.
>>> help(bytes.join)
Help on method_descriptor:
join(...)
 B.join(iterable_of_bytes) -> bytes
 Concatenate any number of bytes objects, with B in between each pair.
### Above sentence should read "Concatenate any number of
bytes/bytearray objects, with B in between each pair, and return the
result as a new bytes object."
 Example: b'.'.join([b'ab', b'pq', b'rs']) -> b'ab.pq.rs'.

I am puzzled as to what you think is missing in the manual.
"Bytes and Byte Array Methods
Bytes and bytearray objects, being "strings of bytes", have all methods
found on strings, with the exception of encode(), format() and
isidentifier(), which do not make sense with these types. ...
Note
The methods on bytes and bytearray objects don’t accept strings as their
arguments, just as the methods on strings don’t accept bytes as their
arguments. For example, 
"
I believe you are right that bytes => bytes/bytearray in the second help
entry.

Terry, you are right. I missed that. My report was based on looking via
the index and finding only "(str method)", no "(byte[sarray] method)".

Closing as "works for me".