git-checkout.txt: clarify that <branch> applies when no path is given.

Otherwise, the sentence "Defaults to HEAD." can be mis-read to mean
that "git checkout -- hello.c" checks-out from HEAD.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -85,9 +85,13 @@ should result in deletion of the path).
 <new_branch>::
 	Name for the new branch.
 
+<tree-ish>::
+	Tree to checkout from (when paths are given). If not specified,
+	the index will be used.
+
 <branch>::
-	Branch to checkout; may be any object ID that resolves to a
-	commit.  Defaults to HEAD.
+	Branch to checkout (when no paths are given); may be any object
+	ID that resolves to a commit.  Defaults to HEAD.
 +
 When this parameter names a non-branch (but still a valid commit object),
 your HEAD becomes 'detached'.