Document regex_sets

[jimav]

The regex_sets feature is mentioned in experimental but, as far as I can find, nowhere documented.

$ find /usr/share/man -name '*perl*' -type f -print0 | xargs -0 zgrep regex_sets
/usr/share/man/man1/perltoc.1.gz:allow aliasing via \f(CW\*(C`\e$x = \e$y\*(C'\fR, \f(CW\*(C`regex_sets\*(C'\fR \- allow extended bracketed
/usr/share/man/man1/perl5260delta.1.gz:Certain regexes making use of the experimental \f(CW\*(C`regex_sets\*(C'\fR feature could
/usr/share/man/man3/experimental.3perl.gz:\&\f(CW\*(C`regex_sets\*(C'\fR \- allow extended bracketed character classes in regexps
/usr/share/man/man3/warnings.3perl.gz:\&         |                 +\- experimental::regex_sets

That was with perl 5.38.2, but I searched for 'regex_sets' in perlre, perlrebackslash, perlrecharclass, and perlreref at https://perldoc.perl.org/5.40.2/perl#Reference-Manual

If the feature is documented anywhere, please add a reference to that place in the pod for experimental (e.g. "See ...") and mention the soon-to-be-no-longer-required use experimental "regex_sets"; at the place so that tag can be used to find it.

[tonycoz]

The PR Dual-Life/experimental#22 adds that link which is the equivalent to https://perldoc.perl.org/perlrecharclass#Extended-Bracketed-Character-Classes