New variable TeX-electric-math.

* tex.el (TeX-math-close-double-dollar): Remove.
(TeX-math-close-single-dollar): Ditto.
(TeX-electric-dollar): Ditto.
(TeX-electric-math): New customizable variable, supersedes
variables above.
(TeX-insert-dollar): Adapt to `TeX-electric-math'.

* doc/auctex.texi (Quotes): Document `TeX-electric-math'.
Document LaTeX-find-matching-begin and LaTeX-find-matching-end.

* doc/auctex.texi (Environments): Document
`LaTeX-find-matching-begin' and `LaTeX-find-matching-end'.