Fix some bugs in c-api/arg.rst documentation

Fix some bugs in c-api/arg.rst documentation

 * replace "the default encoding" by "'utf-8' encoding"
 * fix "w" / "w*" / "w#" doc: similar to "y" / "y*" / "y#"
   and not "s" / "s*" / "s#"
 * "u#": remove "Non-Unicode objects are handled by interpreting their
   read-buffer pointer ...", it's no more true
 * "es", "es#": remove "... and objects convertible to Unicode into a character
   buffer", it's no more true
 * Py_BuildValue(), "K" and "L" formats: specify the name of the C type on
   Windows (_int64 / unsigned _int64) as done for PyArg_Parse*() long long
   types
--CETTE ligne, et les suivantes ci-dessous, seront ignorées--

M    Doc/c-api/arg.rst