Document Speech* apps hangup on failure and suggest TryExec

Document Speech* apps hangup on failure and suggest TryExec

The Speech API apps return -1 on failure, which will hang up the channel. This
may not be desirable behavior for some, but it isn't something that can be
changed without breaking people's dialplans or writing an option to all of the
Speech apps that does what TryExec already does. This patch documents the
hangup behavior of the apps, and suggests TryExec as the solution.

(closes issue AST-813)
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Merged revisions 362815 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@362816 65c4cc65-6c06-0410-ace0-fbb531ad65f3