Update RefTeX manual

Update RefTeX manual

* doc/misc/reftex.texi: (Builtin Label Environments): Mention
supported LaTeX packages ctable, minted and listings.
(Reference Styles): Update the supported reference styles and the
corresponding macros.
(Citation Styles): Mention support for biblatex package.
(Language Support): Describe usage of non-ASCII characters in the
LaTeX \label definition.
(Style Files): Mention biblatex.el as well.
(Options - Defining Label Environments): Add entry for
`reftex-label-regexps' option.
Other minor improvements.

(cherry picked from commit 8d687695c528e4edb04cca5602470d349e333258)