Explain the empty req_uri branch. Props duck_. fixes #17177

git-svn-id: https://develop.svn.wordpress.org/trunk@18467 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/class-wp.php

@@ -186,6 +186,7 @@ class WP {
 			// Look for matches.
 			$request_match = $request;
 			if ( empty( $req_uri ) ) {
+				// An empty request could only match against ^$ regex
 				if ( isset( $rewrite['$'] ) ) {
 					$this->matched_rule = '$';
 					$query = $rewrite['$'];