provide completion for options in `TeX-arg-document'

* latex.el (LaTeX-global-class-files): New variable.
(TeX-arg-document): Provide completion for class options, based on
`LaTeX-arg-usepackage'.  Use `LaTeX-global-class-files'.
(LaTeX-style-list): Mention that if `TeX-arg-input-file-search' is
set to `t' this variable will be ignored.

* tex.el (TeX-normal-mode): Reset `LaTeX-global-class-files' when
ARG is non-nil.

* style/article.el (LaTeX-article-class-options): New variable.

* style/book.el (LaTeX-book-class-options): New variable.

* style/report.el (LaTeX-report-class-options): New variable.
improve completion in LaTeX-arg-usepackage

* latex.el (LaTeX-provided-class-options-member): Fix typo.
(LaTeX-arg-usepackage): Provide completion for more than one
package in mandatory argument.
Minor docfix.

* latex.el (LaTeX-style-list): Correct docstring.