Replace obsolete subfigure package with subfig

Replace obsolete subfigure package with subfig

* doc/misc/reftex.texi (Builtin Label Environments): Mention the
subfig LaTeX package instead of the obsolete subfigure.

* lisp/textmodes/reftex-vars.el (reftex-label-alist-builtin):
Replace obsolete subfigure entry with subfig.  Adjust the macro
names accordingly and the regexp for matching the context.
Unify the descriptions of all entries.
(reftex-default-label-alist-entries): Use subfig as well.

(cherry picked from commit 5c3deffd9f4647e0dff33ffc84a28eb6fedb3012)