bridge_softmix.c: Made use ao2_replace() instead of the inline equivalent.

* Clarified some read/write format comments.

* Fixed a doxygen tag typo.
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Merged revisions 423423 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423424 65c4cc65-6c06-0410-ace0-fbb531ad65f3