Add deprecation notice for 4.6 and update docs

docs/en/02_Developer_Guides/03_Forms/01_Validation.md

@@ -274,9 +274,10 @@ class MyController extends Controller
 ### Validation in the CMS
 
 In the CMS, we're not creating the forms for editing CMS records. The `Form` instance is generated for us so we cannot
-call `setValidator` easily. However, a `DataObject` can provide its' own `Validator` instance through the 
-`getCMSValidator()` method. The CMS interfaces such as [LeftAndMain](api:SilverStripe\Admin\LeftAndMain), [ModelAdmin](api:SilverStripe\Admin\ModelAdmin) and [GridField](api:SilverStripe\Forms\GridField\GridField) will 
-respect the provided `Validator` and handle displaying error and success responses to the user. 
+call `setValidator` easily. However, a `DataObject` can provide its' own `Validator` instance/s through the 
+`getValidatorList()` method. The CMS interfaces such as [LeftAndMain](api:SilverStripe\Admin\LeftAndMain),
+[ModelAdmin](api:SilverStripe\Admin\ModelAdmin) and [GridField](api:SilverStripe\Forms\GridField\GridField) will 
+respect the provided `Validator`/s and handle displaying error and success responses to the user. 
 
 [info]
 Again, custom error messages can be provided through the `FormField`

src/ORM/DataObject.php

@@ -2457,6 +2457,11 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
 
         // Support for the old method during the deprecation period
         if ($this->hasMethod('getCMSValidator')) {
+            Deprecation::notice(
+                '4.6',
+                'getCMSValidator() is removed in 5.0 in favour of getValidatorList()'
+            );
+
             $validatorList->addValidator($this->getCMSValidator());
         }