Overall, this looks good.  Some mostly minor comments in this review.

http://codereview.appspot.com/63060/diff/1/2
File Doc/library/zlib.rst (right):

http://codereview.appspot.com/63060/diff/1/2#newcode136
Line 136: When specified, it will cause the function's return value to
be a (uncompressed, offset)
how about this wording:

..."to be a tuple of (uncompressed, offset), with the"...

http://codereview.appspot.com/63060/diff/1/2#newcode142
Line 142: Added the *offset* argument
missing . at the end of the sentence.  Also, mention that this function
now accepts keyword arguments.

http://codereview.appspot.com/63060/diff/1/3
File Lib/test/test_zlib.py (right):

http://codereview.appspot.com/63060/diff/1/3#newcode110
Line 110: c = zlib.compress(a)+zlib.compress(b)
please surround the + with spaces for readability and consistency with
other code in the file.\

http://codereview.appspot.com/63060/diff/1/4
File Modules/zlibmodule.c (right):

http://codereview.appspot.com/63060/diff/1/4#newcode193
Line 193: "the start position in the string to start decompresion and,
if spceified,\n"
typo: specified.

http://codereview.appspot.com/63060/diff/1/4#newcode296
Line 296: offset = length-zst.avail_in+offset;
leave spaces around your - and + operators and this becomes easier to
read.

http://codereview.appspot.com/63060