[Map][Google] Add documentation about ux_map.google_maps.default_map_id

Author: Kocal

src/Map/doc/index.rst

@@ -34,6 +34,12 @@ Configuration is done in your ``config/packages/ux_map.yaml`` file:
     # config/packages/ux_map.yaml
     ux_map:
         renderer: '%env(resolve:default::UX_MAP_DSN)%'
+        
+        # Google Maps specific configuration
+        google_maps:
+            # Configure the default Map Id (https://developers.google.com/maps/documentation/get-map-id),
+            # without to manually configure it in each map instance (through "new GoogleOptions(mapId: 'your_map_id')").
+            default_map_id: null
 
         # Google Maps specific configuration
         google_maps: