Issue #8711: Document PyUnicode_DecodeFSDefault*() functions

 * Add paragraph titles to c-api/unicode.rst.
 * Fix PyUnicode_DecodeFSDefault*() comment: it now uses the "surrogateescape"
   error handler (and not "replace")
 * Remove "The function is intended to be used for paths and file names only
   during bootstrapping process where the codecs are not set up." from
   PyUnicode_FSConverter() comment: it is used after the bootstrapping and for
   other purposes than file names
2 files changed