setup: document prefix

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

setup.c

@@ -519,6 +519,12 @@ int check_repository_format(void)
 	return check_repository_format_gently(NULL);
 }
 
+/*
+ * Returns the "prefix", a path to the current working directory
+ * relative to the work tree root, or NULL, if the current working
+ * directory is not a strict subdirectory of the work tree root. The
+ * prefix always ends with a '/' character.
+ */
 const char *setup_git_directory(void)
 {
 	const char *retval = setup_git_directory_gently(NULL);