contrib/subtree: fix "unsure" for --message in the document

Revise the documentation and remove previous "unsure" after making sure
that --message supports only 'add', 'merge', 'pull', and 'split --rejoin'.

Signed-off-by: Danny Lin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: danny0838

contrib/subtree/git-subtree.txt

@@ -139,7 +139,7 @@ OPTIONS
 
 -m <message>::
 --message=<message>::
-	This option is only valid for add, merge and pull (unsure).
+	This option is only valid for add, merge, pull, and split --rejoin.
 	Specify <message> as the commit message for the merge commit.