dahdi.conf.sample: Add description for "buffers" setting.

This contains an edited version of the patch originally created by John
Bigelow.

(closes issue ASTERISK-14435)
Reported by: John Bigelow
Patches:
      buffers.patch (license #5091) patch uploaded by John Bigelow
      0001-dahdi.conf.sample-Add-description-for-buffers-settin.patch (license #5417) patch uploaded by Shaun Ruffell
      Modified
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Merged revisions 374727 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 374728 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 374729 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374730 65c4cc65-6c06-0410-ace0-fbb531ad65f3