Add a bit of documentation on this code, including pointers

to relevant documents and comment on timing issues.

Initial merge of the code in

	http://bugs.digium.com/view.php?id=8586

by Filippo Grassilli (Hyppo) to support
the SMS Protocol 2.

In this commit i have tried to minimize the diffs, so further
code cleanup will come in subsequent commits.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48736 65c4cc65-6c06-0410-ace0-fbb531ad65f3