[docs] simplify int() signature docs (issue 16036)

[storchaka at gmail.com]

http://bugs.python.org/review/16036/diff/6073/Doc/library/functions.rst
File Doc/library/functions.rst (right):

http://bugs.python.org/review/16036/diff/6073/Doc/library/functions.rst#newcode639
Doc/library/functions.rst:639: If *base* is given, then *x* must be a
string.  The string must be
Somewhere should be exposed that x must be str, bytes, bytearray or a
subclass.

http://bugs.python.org/review/16036/