Merge r243800 from head to stable/9:

Merge r243800 from head to stable/9:

  Specifically point at the Handbook instructions for world updates in
  UPDATING by URL.

  As there has been some confusion over the need to run "mergemaster -p",
  part of our standard upgrade procedure, following the recent addition of
  an "auditdistd" user, add a note about it to UPDATING explicitly.

git-svn-id: svn://svn.freebsd.org/base/stable/9@244393 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f