Rename LaTeX-auto-insert-label and document its use for sections.

* latex.el (LaTeX-insert-label): Rename from
`LaTeX-auto-insert-label' and mention sections in the doc-string.
(LaTeX-label): Update accordingly.

* doc/auctex.texi (Sectioning): Mention `LaTeX-insert-label'.
(Environments): Update `LaTeX-insert-label' name and document use
for sectioning commands.
(Environments, Completion): Prettify
`TeX-complete-expert-commands' documentation by using a table
environment.

* doc/changes.texi: Rename `LaTeX-auto-insert-label' to
`LaTeX-insert-label' and mention sections too.