Add an "instructions" key to custom fields

Summary: Ref T1049. I'm planning to use this in Harbormaster custom fields shortly.

Test Plan: {F133843}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D8606

src/docs/user/configuration/custom_fields.diviner

@@ -101,6 +101,8 @@ When defining custom fields using a configuration option like
   - **default**: Default field value.
   - **strings**: Allows you to override specific strings based on the field
     type. See below.
+  - **instructions**: Optional block of remarkup text which will appear
+    above the control when rendered on the edit view.
 
 The `strings` value supports different strings per control type. They are:
 

src/infrastructure/customfield/field/PhabricatorCustomField.php

@@ -1060,6 +1060,17 @@ abstract class PhabricatorCustomField {
   }
 
 
+  /**
+   * @task edit
+   */
+  public function getInstructionsForEdit() {
+    if ($this->proxy) {
+      return $this->proxy->getInstructionsForEdit();
+    }
+    return null;
+  }
+
+
   /**
    * @task edit
    */

src/infrastructure/customfield/field/PhabricatorCustomFieldList.php

@@ -108,6 +108,12 @@ final class PhabricatorCustomFieldList extends Phobject {
 
     foreach ($enabled as $field_key => $field) {
       $field_handles = array_select_keys($handles, $phids[$field_key]);
+
+      $instructions = $field->getInstructionsForEdit();
+      if (strlen($instructions)) {
+        $form->appendRemarkupInstructions($instructions);
+      }
+
       $form->appendChild($field->renderEditControl($field_handles));
     }
   }

src/infrastructure/customfield/standard/PhabricatorStandardCustomField.php

@@ -211,6 +211,10 @@ abstract class PhabricatorStandardCustomField
     $this->setFieldValue($value);
   }
 
+  public function getInstructionsForEdit() {
+    return $this->getFieldConfigValue('instructions');
+  }
+
   public function renderEditControl(array $handles) {
     return id(new AphrontFormTextControl())
       ->setName($this->getFieldKey())