Document TeX-add-style-hook's new 3rd argument.

* latex.el (LaTeX-dialect): Correct doctstring for consistency
with definition of function TeX-add-style-hook.

* doc/auctex.texi (Simple Style): modify example with
'TeX-add-style-hook', so that 3rd argument is added. Modify @defun
of 'TeX-add-style-hook' also to document addition of 3rd argument.

Signed-off-by: Tassilo Horn <tsdh@gnu.org>