more documentation of data structure and functions.

more documentation of data structure and functions.
Of interest:
+ ast_get_manager_by_name_locked() is now without the ast_
  prefix as it is a local function;

+ unuse_eventqent() renamed to unref_event(), and returns
  the pointer to the next entry.

+ marked with XXX a couple of usages of unref_event()
  because i suspect we are addressing the wrong entry.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@45648 65c4cc65-6c06-0410-ace0-fbb531ad65f3