Create hook_civicrm_alterAngular.md

See https://github.com/civicrm/civicrm-core/pull/10085
@totten I've added docs for this hook.

docs/framework/angular/changeset.md

@@ -19,7 +19,11 @@ independent deployments with independent configurations).
 
 ...
 
-## tldr
+## Hooks
+
+* **[hook_civicrm_alterAngular](/hooks/hook_civicrm_alterAngular.md)** - Alter the definition of some Angular HTML partials.
+
+## Example
 
 ```php
 function mailwords_civicrm_alterAngular(\Civi\Angular\Manager $angular) {

docs/hooks/hook_civicrm_alterAngular.md

+# hook_civicrm_alterAngular
+
+## Summary
+
+Alter the definition of some Angular HTML partials; allows the injection of [Changesets](/framework/angular/changeset/).
+
+## Availability
+
+This hook is available in CiviCRM 4.7.21 and later.
+
+## Definition
+
+     hook_civicrm_alterAngular(&$angular)
+
+## Parameters
+
+- array `$angular` - `\Civi\Angular\Manager`
+
+## Example
+
+```php
+function example_civicrm_alterAngular($angular) {
+  $angular->add(\Civi\Angular\ChangeSet::create('mychanges')
+    ->alterHtml('~/crmMailing/EditMailingCtrl/2step.html', function(phpQueryObject $doc) {
+      $doc->find('[ng-form="crmMailingSubform"]')->attr('cat-stevens', 'ts(\'wild world\')');
+    })
+  );
+}
+```