backout: make help more explicit about what backout does

The help for backout explains:

   The backout command merges the reverse effect of the reverted
   changeset into the working directory.

Unfortunately, that does not make it obvious to a newcomer what the
backout command does.  Since it performs a 3-way merge, what is the
common ancestor?  Will the result be automatically committed?  What is
this reverted changeset --- is it the rev passed with -r on the
command line or its inverse?

So try to clarify the description, avoiding jargon and being
explicit about what happens from the user's perspective.

Thanks to Gilles Moris, Steve Borho, Kevin Bullock, and timeless for
help.

mercurial/commands.py

@@ -204,26 +204,22 def archive(ui, repo, dest, **opts):
 def backout(ui, repo, node=None, rev=None, **opts):
     '''reverse effect of earlier changeset
 
-    The backout command merges the reverse effect of the reverted
-    changeset into the working directory.
-
-    With the --merge option, it first commits the reverted changes
-    as a new changeset. This new changeset is a child of the reverted
-    changeset.
-    The --merge option remembers the parent of the working directory
-    before starting the backout, then merges the new head with that
-    changeset afterwards.
-    This will result in an explicit merge in the history.
-
-    If you backout a changeset other than the original parent of the
-    working directory, the result of this merge is not committed,
-    as with a normal merge. Otherwise, no merge is needed and the
-    commit is automatic.
-
-    Note that the default behavior (without --merge) has changed in
-    version 1.7. To restore the previous default behavior, use
-    :hg:`backout --merge` and then :hg:`update --clean .` to get rid of
-    the ongoing merge.
+    Prepare a new changeset with the effect of REV undone in the
+    current working directory.
+
+    If REV is the parent of the working directory, then this changeset
+    is committed automatically. Otherwise, hg needs to merge the
+    changes and the merged result is left uncommitted.
+
+    By default, the pending changeset will have one parent,
+    maintaining a linear history. With --merge, the pending changeset
+    will instead have two parents: the old parent of the working
+    directory and a child of REV that simply undoes REV.
+
+    Before version 1.7, the default behavior was equivalent to
+    specifying --merge followed by :hg:`update --clean .` to cancel
+    the merge and leave the child of REV as a head to be merged
+    separately.
 
     See :hg:`help dates` for a list of formats valid for -d/--date.