Widgets: Rename "Custom Menu" widget to "Navigation Menu".

Props gk.loveweb, ChrisHardie, ajayghaghretiya1, melchoyce.
Fixes #40442.

Built from https://develop.svn.wordpress.org/trunk@41868


git-svn-id: http://core.svn.wordpress.org/trunk@41702 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/customize-selective-refresh.js

@@ -883,7 +883,7 @@ wp.customize.selectiveRefresh = ( function( $, api ) {
 			/*
 			 * Only trigger renders on (nested) partials that have been not been
 			 * handled yet. An example where this would apply is a nav menu
-			 * embedded inside of a custom menu widget. When the widget's title
+			 * embedded inside of a navigation menu widget. When the widget's title
 			 * is updated, the entire widget will re-render and then the event
 			 * will be triggered for the nested nav menu to do any initialization.
 			 */