Show an example of deleting commits with git-rebase.

This particular use of git-rebase to remove a single commit or a
range of commits from the history of a branch recently came up on
the mailing list.  Documenting the example should help other users
arrive at the same solution on their own.

It also was not obvious to the newcomer that git-rebase is able to
accept any commit for --onto <newbase> and <upstream>.  We should
at least minimally document this, as much of the language in
git-rebase's manpage refers to 'branch' rather than 'committish'.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-rebase.txt

@@ -114,6 +114,27 @@ would result in:
 
 This is useful when topicB does not depend on topicA.
 
+A range of commits could also be removed with rebase.  If we have
+the following situation:
+
+------------
+    E---F---G---H---I---J  topicA
+------------
+
+then the command
+
+    git-rebase --onto topicA~5 topicA~2 topicA
+
+would result in the removal of commits F and G:
+
+------------
+    E---H'---I'---J'  topicA
+------------
+
+This is useful if F and G were flawed in some way, or should not be
+part of topicA.  Note that the argument to --onto and the <upstream>
+parameter can be any valid commit-ish.
+
 In case of conflict, git-rebase will stop at the first problematic commit
 and leave conflict markers in the tree.  You can use git diff to locate
 the markers (<<<<<<) and make edits to resolve the conflict.  For each
@@ -141,10 +162,12 @@ OPTIONS
 <newbase>::
 	Starting point at which to create the new commits. If the
 	--onto option is not specified, the starting point is
-	<upstream>.
+	<upstream>.  May be any valid commit, and not just an
+	existing branch name.
 
 <upstream>::
-	Upstream branch to compare against.
+	Upstream branch to compare against.  May be any valid commit,
+	not just an existing branch name.
 
 <branch>::
 	Working branch; defaults to HEAD.