Adjust feature tooltip message when OSM layer is hidden

(re: https://github.com/openstreetmap/iD/issues/4292#issuecomment-330588097)

data/core.yaml

@@ -397,6 +397,7 @@ en:
     fill_area: Fill Areas
     map_features: Map Features
     autohidden: "These features have been automatically hidden because too many would be shown on the screen.  You can zoom in to edit them."
+    osmhidden: "These features have been automatically hidden because the OpenStreetMap layer is hidden."
   feature:
     points:
       description: Points

dist/locales/en.json

@@ -488,7 +488,8 @@
             },
             "fill_area": "Fill Areas",
             "map_features": "Map Features",
-            "autohidden": "These features have been automatically hidden because too many would be shown on the screen.  You can zoom in to edit them."
+            "autohidden": "These features have been automatically hidden because too many would be shown on the screen.  You can zoom in to edit them.",
+            "osmhidden": "These features have been automatically hidden because the OpenStreetMap layer is hidden."
         },
         "feature": {
             "points": {

modules/ui/map_data.js

@@ -335,7 +335,8 @@ export function uiMapData(context) {
                             key = (d === 'wireframe' ? t('area_fill.wireframe.key') : null);
 
                         if (name === 'feature' && autoHiddenFeature(d)) {
-                            tip += '<div>' + t('map_data.autohidden') + '</div>';
+                            var msg = showsLayer('osm') ? t('map_data.autohidden') : t('map_data.osmhidden');
+                            tip += '<div>' + msg + '</div>';
                         }
                         return uiTooltipHtml(tip, key);
                     })