Correct documentation for ModuleLoad AMI action

The documentation incorrectly listed 'rtp' as a reloadable subsystem
and left out many other reloadable subsystems. It is now also
documented that subsystems may only be reloaded, not loaded or
unloaded.

(closes issue AST-977)
Reported-by: John Bigelow
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Merged revisions 372354 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 372358 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@372365 65c4cc65-6c06-0410-ace0-fbb531ad65f3