Construct socket name, according to the Postgres docs, and document as such.

(closes issue #17392)
 Reported by: dps
 Patches: 
       20100525__issue17392.diff.txt uploaded by tilghman (license 14)
 Tested by: dps


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@265894 65c4cc65-6c06-0410-ace0-fbb531ad65f3