Documentation: A...B shortcut for checkout and rebase

Describe the A...B shortcuts for checkout and rebase [-i] which were
introduced in these commits:

619a64e ("checkout A...B" switches to the merge base between A and B, 2009-10-18)
61dfa1b ("rebase --onto A...B" replays history on the merge base between A and B, 2009-11-20)
230a456 (rebase -i: teach --onto A...B syntax, 2010-01-07)

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -136,6 +136,10 @@ edits from your current working tree.
 As a special case, the `"@\{-N\}"` syntax for the N-th last branch
 checks out the branch (instead of detaching).  You may also specify
 `-` which is synonymous with `"@\{-1\}"`.
++
+As a further 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`.
 
 <new_branch>::
 	Name for the new branch.

Documentation/git-rebase.txt

@@ -206,6 +206,10 @@ OPTIONS
 	--onto option is not specified, the starting point is
 	<upstream>.  May be any valid commit, and not just an
 	existing branch name.
++
+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.
 
 <upstream>::
 	Upstream branch to compare against.  May be any valid commit,