QPID-4817: merged message grouping documentation from trunk to 0.22 branch.
The merge command was:
  svn merge -c 1480196 https://svn.apache.org/repos/asf/qpid/trunk/


git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.22@1480212 13f79535-47bb-0310-9956-ffa450edef68