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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@423068 f38db490-d61c-443f-a65b-d21fe96a405b