Add default tabular-like env for use in tables.

* latex.el (LaTeX-default-tabular-environment): New variable.
(LaTeX-env-figure): Use it instead of hard-coding "tabular".

* style/tabularx.el ("tabularx"): Set
`LaTeX-default-tabular-environment' to "tabularx".
Document TeX-source-correlate-method.

* doc/auctex.texi (Processor Options): Document
`TeX-source-correlate-method'.