git-tar-tree: complete deprecation conversion message

The syntax for git-archive is different; warn about it in the
deprecation message on the manual page.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-tar-tree.txt

@@ -13,7 +13,7 @@ SYNOPSIS
 DESCRIPTION
 -----------
 THIS COMMAND IS DEPRECATED.  Use `git-archive` with `--format=tar`
-option instead.
+option instead (and move the <base> argument to `--prefix=base/`).
 
 Creates a tar archive containing the tree structure for the named tree.
 When <base> is specified it is added as a leading path to the files in the