Need to update documentation to reflect export block syntax

[Mark Edgington <edgimar@gmail.com>]

I recently updated my org-mode git repository (for a long time I've
had an older git version that I was using), and I am now having
problems when doing latex exports.

I bisected with git to find that the "problem" starts with this revision:

commit 54318add34f09ff39d3fd034a4c1a89f60fd8759
Author: Nicolas Goaziou <mail@nicolasgoaziou.fr>
Date:   Tue Feb 3 16:16:54 2015 +0100

    Change export block syntax

It seems that #+begin_latex is no longer supported, and that when
using it, it results in escaped latex comment symbols (e.g. \% in the
tex file instead of just %), as well as \begin{latex} and \end{latex}
in the exported code.

If it is no longer supported, then it is important that the org-mode
documentation be updated (see e.g.
http://orgmode.org/manual/Quoting-LaTeX-code.html ), since those using
the latest release will have trouble exporting using the old syntax.