Documentation: read-tree --prefix works with existing subtrees

Since 34110cd4 (Make 'unpack_trees()' have a separate source and
destination index) it is no longer true that a subdirectory with
the same prefix must not exist.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-read-tree.txt

@@ -83,11 +83,10 @@ OPTIONS
 
 --prefix=<prefix>/::
 	Keep the current index contents, and read the contents
-	of the named tree-ish under the directory at `<prefix>`. The
+	of the named tree-ish under the directory at `<prefix>`.
-	original index file cannot have anything at the path
+	The command will refuse to overwrite entries that already
-	`<prefix>` itself, nor anything in the `<prefix>/`
+	existed in the original index file. Note that the `<prefix>/`
-	directory.  Note that the `<prefix>/` value must end
+	value must end with a slash.
-	with a slash.
 
 --exclude-per-directory=<gitignore>::
 	When running the command with `-u` and `-m` options, the