Distinguish common and advanced macros/envs.

* tex.el (TeX-add-advanced-macros/envs): New defcustom.
(TeX-add-advanced-macros/envs-p): New function.

* doc/auctex.texi: Document addition of common vs. advanced macros
and environments both for users and style file authors.

* doc/changes.texi: Give a hint to the above.