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

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