Add narrowing commands.

* latex.el (LaTeX-narrow-to-environment): New function, disabled
by default.
(LaTeX-common-initialization): Add key binding for
`LaTeX-narrow-to-environment'.

* tex.el (VirTeX-common-initialization): Add key binding for
`TeX-narrow-to-group'.
(TeX-narrow-to-group): New function, disabled by default.

* doc/auctex.texi (Narrowing): Document narrowing commands.

* doc/changes.texi: Mention narrowing commands.