[docs] Sync-up docstrings in C version of the the decimal module (issue 27779)

[stefan at bytereef.org]

http://bugs.python.org/review/27779/diff/18450/Modules/_decimal/docstrings.h
File Modules/_decimal/docstrings.h (right):

http://bugs.python.org/review/27779/diff/18450/Modules/_decimal/docstrings.h#newcode405
Modules/_decimal/docstrings.h:405: Returns a copy of the operand with
the sign set to 0. This operation is unaffected by\n\
I think "sign set to zero" is a bit confusing for the C version.
"absolute value" sounds fine to me.

http://bugs.python.org/review/27779/