Document the need to purge caches after updating differential.generated-paths

Summary: Fixes T6378.

Test Plan: Set config to `/.*/`, created a new diff, everything was collapsed as generated.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6378

Differential Revision: https://secure.phabricator.com/D12159

src/applications/differential/config/PhabricatorDifferentialConfigOptions.php

@@ -20,6 +20,8 @@ final class PhabricatorDifferentialConfigOptions
   }
 
   public function getOptions() {
+    $caches_href = PhabricatorEnv::getDoclink('Managing Caches');
+
     $custom_field_type = 'custom:PhabricatorCustomFieldConfigOptionType';
 
     $fields = array(
@@ -123,8 +125,14 @@ final class PhabricatorDifferentialConfigOptions
         ->setDescription(
           pht(
             'List of file regexps that should be treated as if they are '.
-            'generated by an automatic process, and thus get hidden by '.
-            'default in differential.'))
+            'generated by an automatic process, and thus be hidden by '.
+            'default in Differential.'.
+            "\n\n".
+            'NOTE: This property is cached, so you will need to purge the '.
+            'cache after making changes if you want the new configuration '.
+            'to affect existing revisions. For instructions, see '.
+            '**[[ %s | Managing Caches ]]** in the documentation.',
+            $caches_href))
         ->addExample("/config\.h$/\n#/autobuilt/#", pht('Valid Setting')),
       $this->newOption('differential.sticky-accept', 'bool', true)
         ->setBoolOptions(