Add a tooltip to the plus button in the "All relations" section (re: #3812)

data/core.yaml

@@ -474,6 +474,7 @@ en:
     all_tags: All tags
     all_members: All members
     all_relations: All relations
+    add_to_relation: Add to a relation
     new_relation: New relation...
     choose_relation: Choose a parent relation
     role: Role

dist/locales/en.json

@@ -584,6 +584,7 @@
             "all_tags": "All tags",
             "all_members": "All members",
             "all_relations": "All relations",
+            "add_to_relation": "Add to a relation",
             "new_relation": "New relation...",
             "choose_relation": "Choose a parent relation",
             "role": "Role",

modules/ui/raw_membership_editor.js

@@ -3,7 +3,7 @@ import {
     select as d3_select
 } from 'd3-selection';
 
-import { t } from '../util/locale';
+import { t, textDirection } from '../util/locale';
 
 import {
     actionAddEntity,
@@ -17,6 +17,7 @@ import { osmEntity, osmRelation } from '../osm';
 import { services } from '../services';
 import { svgIcon } from '../svg';
 import { uiCombobox, uiDisclosure } from './index';
+import { tooltip } from '../util/tooltip';
 import { utilArrayGroupBy, utilDisplayName, utilNoAuto, utilHighlightEntities } from '../util';
 
 
@@ -308,10 +309,14 @@ export function uiRawMembershipEditor(context) {
                 .append('div')
                 .attr('class', 'add-row');
 
-            addRowEnter
+            var addRelationButton = addRowEnter
                 .append('button')
-                .attr('class', 'add-relation')
+                .attr('class', 'add-relation');
+
+            addRelationButton
                 .call(svgIcon('#iD-icon-plus', 'light'));
+            addRelationButton
+                .call(tooltip().title(t('inspector.add_to_relation')).placement(textDirection === 'ltr' ? 'right' : 'left'));
 
             addRowEnter
                 .append('div')