manager: Clarify eventfilter documentation. Textual changes only.

Review: https://reviewboard.asterisk.org/r/3133/
........

Merged revisions 406079 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 406080 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 406081 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@406082 65c4cc65-6c06-0410-ace0-fbb531ad65f3