I'm happy to prepare a patch in this case. I would simply remove all mention of syntax tables, as that functionality is no longer available.
Attached. Here's the commit message to explain what I've done:
Remove mention of both Emacs and non-Emacs syntax tables, as these are no
longer supported by the code; instead, fixed character classes are used.
Document the word character class (alnum + _).
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”.
Leave a TODO to document the classes that can be used with \s and \S. (This
was not previously documented, and is best left to another commit.)
0001-doc-regex.texi-remove-Emacs-specific-documentation-m.patch