Add documentation for setting username/password in SIP dial string.

(closes issue #11587)
Reported by: sobomax
Patches:
      dialstring_doc.diff uploaded by sobomax (license 359)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@105378 65c4cc65-6c06-0410-ace0-fbb531ad65f3