Don't perform an XInclude to a document node that may not always be present

Because some of the manager events are defined in the top of the source, due
to the macro calls not containing all necessary information to have the
documentation colocated with the call itself, several include statements were
failing when built with 'make'.  While this did not cause any problems in
compilation or validation, it did result in a number of warnings being dumped
to stderr.

This patch changes those references such that they always resolve, regardless
of the documentation build options.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369939 65c4cc65-6c06-0410-ace0-fbb531ad65f3