Note that CDRs are immutable once a bridge is torn down

CDRs cannot be modified after a bridge is torn down, (e.g. after
Dial() returns) even though the CDR() function may be called. Since
modifying the CDR code to change this behavior could very easily
break all kinds of things, this patch just documents this limitation.

(closes issues ASTERISK-16923)
Review: https://reviewboard.asterisk.org/r/1720/
........

Merged revisions 354749 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 354750 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@354751 65c4cc65-6c06-0410-ace0-fbb531ad65f3