chan_sip: Clarify that sipdebug=yes cannot be undone by the CLI.

chan_sip: Clarify that sipdebug=yes cannot be undone by the CLI.

Document it in sip.conf.

ASTERISK-24249 #close
Reported by: Avinash Mohod

Review: https://reviewboard.asterisk.org/r/3926/
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Merged revisions 423066 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 423067 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 423068 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@423069 65c4cc65-6c06-0410-ace0-fbb531ad65f3