Allow to include sections of other parts of the xml documentation.

Avoid duplicating xml documentation by allowing to include other parts of
the xml documentation using XInclude.
Example:
   <xi:include xpointer="xpointer(/docs/function[@name='CHANNEL']/synopsis)" />
(Insert this line to include the synopsis of the CHANNEL function xml
documentation).

It is also possible to include documentation from other files in the
'documentation/' directory using the href="" attribute inside a xinclude
element.

(closes issue #15107)
Reported by: lmadsen

(issue #14444)
Reported by: ewieling



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@194982 65c4cc65-6c06-0410-ace0-fbb531ad65f3