Fix another incorrect case with meetme's PIN logic and add documentation

This fixes an issue where a user of a dynamic conference was asked for a PIN
twice.  This also adds documentation to assist in future modifications to the
piece of code responsible for PIN checking.

(closes issue AST-670)
........

Merged revisions 344439 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 344440 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@344441 65c4cc65-6c06-0410-ace0-fbb531ad65f3