doc: remove Emacs-specific documentation; match code
* doc/regex.texi: Remove mention of both Emacs and non-Emacs syntax tables,
as these are no longer supported by the code. Document the word character
class (alnum + _). Add documentation for \s and \S. Replace mentions
of #defining emacs with RE_NO_GNU_OPS (which takes effect in the opposite
sense); merge the node “GNU Emacs Operators” into “GNU Operators”. For \`
and \', refer to the “whole string” rather than the (Emacs) “buffer”.