chore: add in note at top of files ready for deletion in v4

src/socket.io/categories.js

@@ -1,5 +1,9 @@
 'use strict';
 
+/**
+ * v4 note — all socket.io methods here have been deprecated, and can be removed for v4
+ */
+
 const categories = require('../categories');
 const user = require('../user');
 const topics = require('../topics');

src/socket.io/categories/search.js

@@ -1,5 +1,9 @@
 'use strict';
 
+/**
+ * v4 note — all socket.io methods here have been deprecated, and can be removed for v4
+ */
+
 const sockets = require('..');
 const api = require('../../api');