adds forceAliasCreations clarification to auth docs (#5014)

* adds forceAliasCreations clarification to auth docs

Author: helgabalashova

src/fragments/lib/auth/js/emailpassword.mdx

@@ -68,6 +68,24 @@ async function confirmSignUp() {
 }
 ```
 
+As part of the options parameter, you can specify the `forceAliasCreation` boolean. By default
+set to `True`. If set to `False` and the email or phone number used for sign up confirmation
+already exists as an alias for a different user, the API will throw an `AliasExistsException`
+error. If set to `True`, the API call will migrate the alias from the previous user to 
+a newly created user.
+
+```js
+import { Auth } from 'aws-amplify';
+
+async function confirmSignUp() {
+    try {
+      await Auth.confirmSignUp(username, code, { forceAliasCreation: false });
+    } catch (error) {
+        console.log('error confirming sign up', error);
+    }
+}
+```
+
 ### Auto sign in after sign up
 
 If you enabled `autoSignIn`, the `sign up` function will dispatch `autoSignIn` hub event after successful confirmation.