branch: clarify <oldbranch> term

Since 52d59cc (branch: add a --copy (-c) option to go with --move
(-m), 2017-06-18) <oldbranch> is used in more operations than just -m.

Let's also clarify what we do if <oldbranch> is omitted.

Signed-off-by: Rubén Justo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rjusto

Documentation/git-branch.txt

@@ -312,7 +312,8 @@ superproject's "origin/main", but tracks the submodule's "origin/main".
 	option is omitted, the current HEAD will be used instead.
 
 <oldbranch>::
-	The name of an existing branch to rename.
+	The name of an existing branch.  If this option is omitted,
+	the name of the current branch will be used instead.
 
 <newbranch>::
 	The new name for an existing branch. The same restrictions as for