[NaCl Docs] Add some messaging system docs for how to use pp::VarDictionaries.


BUG=none
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/23688008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225100 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225100