doc: git-rebase: move --onto explanation down

There's a very clear explanation with examples of using --onto which is
currently buried in the very long DESCRIPTION section. This moves it to
its own section, so that we can reference the explanation from the
`--onto` option by name.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jvns

Documentation/git-rebase.adoc

@@ -114,6 +114,9 @@ will result in:
     D---E---A'---F master
 ------------
 
+TRANSPLANTING A TOPIC BRANCH WITH --ONTO
+----------------------------------------
+
 Here is how you would transplant a topic branch based on one
 branch to another, to pretend that you forked the topic branch
 from the latter branch, using `rebase --onto`.
@@ -240,6 +243,8 @@ As a special case, you may use "A\...B" as a shortcut for the
 merge base of A and B if there is exactly one merge base. You can
 leave out at most one of A and B, in which case it defaults to HEAD.
 
+See TRANSPLANTING A TOPIC BRANCH WITH --ONTO above for examples.
+
 --keep-base::
 	Set the starting point at which to create the new commits to the
 	merge base of `<upstream>` and `<branch>`. Running