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/PasswordReset.js

@@ -84,8 +84,14 @@ export default class PasswordReset {
 
         try {
             await this.client.setPassword({
+                // Note: Though this sounds like a login type for identity servers only, it
+                // has a dual purpose of being used for homeservers too.
                 type: "m.login.email.identity",
+                // TODO: Remove `threepid_creds` once servers support proper UIA
+                // See https://github.com/matrix-org/synapse/issues/5665
+                // See https://github.com/matrix-org/matrix-doc/issues/2220
                 threepid_creds: creds,
+                threepidCreds: creds,
             }, this.password);
         } catch (err) {
             if (err.httpStatus === 401) {

src/async-components/views/dialogs/secretstorage/CreateSecretStorageDialog.js

@@ -201,7 +201,8 @@ export default class CreateSecretStorageDialog extends React.PureComponent {
                     type: 'm.id.user',
                     user: MatrixClientPeg.get().getUserId(),
                 },
-                // https://github.com/matrix-org/synapse/issues/5665
+                // TODO: Remove `user` once servers support proper UIA
+                // See https://github.com/matrix-org/synapse/issues/5665
                 user: MatrixClientPeg.get().getUserId(),
                 password: this.state.accountPassword,
             });

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,
         };