Redocuments sip types peer, user, friend in sip.conf.sample

There was faulty information in the sample config describing user as a synonym for friend
so it has been changed to better elaborate on the differences between the three entity
types.

(closes issue ASTERISK-15537)
Reported by: yarique
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Merged revisions 352511 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 352512 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@352513 65c4cc65-6c06-0410-ace0-fbb531ad65f3