Add the ability to specify technology specific documentation

A number of applications/AMI commands in Asterisk have specific behavioral
differences depending on the resource or channel technology those
applications are executed on.  For example, the MessageSend application/
command is technology agnostic, but how the channel drivers that support
that functionality behave is dependant on the protocols and channel
driver implementation.  Prior to this patch, those details were either
documented in the application/command documentation itself, or were left
undocumented.

This patch adds a new element to the documentation schema, <info/>.  An info
node is essentially a piece of technology specific reference information that
can be included by any top level XML documentation node.  For example, the
MessageSend application can now include XMPP/SIP specific information, where
that technology specific information can be defined in chan_motif/res_xmpp/
chan_sip.  Likewise, that information can also be included in the MessageSend
AMI command.

Review: https://reviewboard.asterisk.org/r/2049




git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370278 65c4cc65-6c06-0410-ace0-fbb531ad65f3