Set inner role="document" in popups (#470)

* Set inner `role="document"` in popups

* Add link to GH issue in comment on why `role="document"` is needed

Author: Malvoz

src/mapml/layers/MapLayer.js

@@ -1184,6 +1184,8 @@ export var MapMLLayer = L.Layer.extend({
 
       popup._container.setAttribute("role", "dialog");
       content.setAttribute("tabindex", "-1");
+      // https://github.com/Maps4HTML/Web-Map-Custom-Element/pull/467#issuecomment-844307818
+      content.setAttribute("role", "document");
       popup._count = 0; // used for feature pagination
 
       if(popup._source._eventParents){ // check if the popup is for a feature or query