Allow standard custom fields to be indexed in global fulltext search

Summary: Fixes T6399. This allows you to use global search to find projects by searching for text in their descriptions.

Test Plan: Added a unique word to a project description, reindexed it, searched, got a hit.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6399

Differential Revision: https://secure.phabricator.com/D10748

src/docs/user/configuration/custom_fields.diviner

@@ -102,6 +102,9 @@ When defining custom fields using a configuration option like
     defaults to `true`). (Note: Empty fields are not shown.)
   - **search**: Show this field on the application's search interface, allowing
     users to filter objects by the field value.
+  - **fulltext**: Index the text in this field as part of the object's global
+    full-text index. This allows users to find the object by searching for
+    the field's contents using global search.
   - **caption**: A caption to display underneath the field (optional).
   - **required**: True if the user should be required to provide a value.
   - **options**: If type is set to **select**, provide options for the dropdown