Doc: document "A...B" form for <tree-ish> in checkout and switch

Using "A...B" has been supported for the <tree-ish> argument for a
while. However, its support has never been explicitly documented.

Explicitly document it so that users know that it is available.

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

Documentation/git-checkout.txt

@@ -350,6 +350,10 @@ leave out at most one of `A` and `B`, in which case it defaults to `HEAD`.
 <tree-ish>::
 	Tree to checkout from (when paths are given). If not specified,
 	the index will be used.
++
+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`.
 
 \--::
 	Do not interpret any more arguments as options.

Documentation/git-restore.txt

@@ -40,6 +40,10 @@ OPTIONS
 +
 If not specified, the contents are restored from `HEAD` if `--staged` is
 given, otherwise from the index.
++
+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`.
 
 -p::
 --patch::