Document breaking change in password rehashing if custom password fie…

…ld (#9901)

* Document breaking change in password rehashing if custom password field

Also added missing likelihood-of-impact, which I'm calling "low" since most folks probably use the default 'password' field name.

* Update upgrade.md

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Co-authored-by: Taylor Otwell <taylor@laravel.com>

upgrade.md

@@ -126,9 +126,15 @@ However, we do **not recommend** that Laravel 10 applications upgrading to Larav
 <a name="password-rehashing"></a>
 #### Password Rehashing
 
-Laravel 11 will automatically rehash your user's passwords during authentication if your hashing algorithm's "work factor" has been updated since the password was last hashed.
+**Likelihood Of Impact: Low**
+
+Laravel 11 will automatically rehash your user's passwords during authentication if your hashing algorithm's "work factor" has been updated since the password was last hashed. 
+
+Typically, this should not disrupt your application; however, if your `User` model's "password" field has a name other than `password`, you should specify the field's name via the model's `authPasswordName` property:
+
+    protected $authPasswordName = 'custom_password_field';
 
-Typically, this should not disrupt your application; however, you may disable this behavior by adding the `rehash_on_login` option to your application's `config/hashing.php` configuration file:
+Alternatively, you may disable password rehashing by adding the `rehash_on_login` option to your application's `config/hashing.php` configuration file:
 
     'rehash_on_login' => false,