Add base XML documentation for res_sip

Thanks to Brad Latus, this patch adds a significant amount much-needed
documentation to res_sip. It should cover all existing configuration
options currently in Asterisk trunk.

Patch-by: Brad Latus (snuffy)
Review: https://reviewboard.asterisk.org/r/2471/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@389148 f38db490-d61c-443f-a65b-d21fe96a405b