document api endpoints (#11958)

* DEV: Documented several group endpoints

* documented some more endpoints

* document more api endpoints

* Document backup endpoints

* remove puts

spec/requests/api/schemas/json/group_add_members_request.json

@@ -0,0 +1,10 @@
+{
+  "additionalProperties": false,
+  "properties": {
+    "usernames": {
+      "type": "string",
+      "description": "comma separated list",
+      "example": "username1,username2"
+    }
+  }
+}