The bridge uniqueid is available for softmix destructor.

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@390956 f38db490-d61c-443f-a65b-d21fe96a405b
Update some doxygen comments.

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@390957 f38db490-d61c-443f-a65b-d21fe96a405b