Fix/document a number of UIA oddities

Edition 2 of https://github.com/matrix-org/matrix-react-sdk/pull/3211
Fixes https://github.com/vector-im/riot-web/issues/13837

src/components/views/auth/InteractiveAuthEntryComponents.js

@@ -538,6 +538,7 @@ export const MsisdnAuthEntry = createReactClass({
                     type: MsisdnAuthEntry.LOGIN_TYPE,
                     // TODO: Remove `threepid_creds` once servers support proper UIA
                     // See https://github.com/vector-im/riot-web/issues/10312
+                    // See https://github.com/matrix-org/matrix-doc/issues/2220
                     threepid_creds: creds,
                     threepidCreds: creds,
                 });

src/components/views/settings/ChangePassword.js

@@ -141,6 +141,12 @@ export default createReactClass({
     _changePassword: function(cli, oldPassword, newPassword) {
         const authDict = {
             type: 'm.login.password',
+            identifier: {
+                type: 'm.id.user',
+                user: cli.credentials.userId,
+            },
+            // TODO: Remove `user` once servers support proper UIA
+            // See https://github.com/matrix-org/synapse/issues/5665
             user: cli.credentials.userId,
             password: oldPassword,
         };