Document support for "limit" in tokenizer-based Custom Fields

Summary:
Fixes T13356. This option is supported and works fine, it just isn't documented.

Add documentation and fix the config option to actually link to it to make life a little easier.

Test Plan: Read documentation.

Maniphest Tasks: T13356

Differential Revision: https://secure.phabricator.com/D20691

src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php

@@ -451,15 +451,20 @@ You can choose the default priority for newly created tasks with
 EOTEXT
       ));
 
+    $fields_description = $this->deformat(pht(<<<EOTEXT
+List of custom fields for Maniphest tasks.
+
+For details on adding custom fields to Maniphest, see [[ %s | %s ]] in the
+documentation.
+EOTEXT
+      ,
+      PhabricatorEnv::getDoclink('Configuring Custom Fields'),
+      pht('Configuring Custom Fields')));
 
     return array(
       $this->newOption('maniphest.custom-field-definitions', 'wild', array())
         ->setSummary(pht('Custom Maniphest fields.'))
-        ->setDescription(
-          pht(
-            'Array of custom fields for Maniphest tasks. For details on '.
-            'adding custom fields to Maniphest, see "Configuring Custom '.
-            'Fields" in the documentation.'))
+        ->setDescription($fields_description)
         ->addExample($fields_json, pht('Valid setting')),
       $this->newOption('maniphest.fields', $custom_field_type, $default_fields)
         ->setCustomData(id(new ManiphestTask())->getCustomFieldBaseClass())

src/docs/user/configuration/custom_fields.diviner

@@ -121,6 +121,10 @@ When defining custom fields using a configuration option like
     supported in text, int and remarkup fields (optional).
   - **copy**: If true, this field's value will be copied when an object is
     created using another object as a template.
+  - **limit**: For control types which use a tokenizer control to let the user
+    select a list of values, this limits how many values can be selected. For
+    example, a "users" field with a limit of "1" will behave like the "Owner"
+    field in Maniphest and only allow selection of a single user.
 
 The `strings` value supports different strings per control type. They are: