Config Projects Extra Fields: link to the doc (like User)

Summary:
Add a documentation link in a specific Config page, that is:

Config > Setting > projects.custom-field-definitions

Comparison example:

| User      | People     |
|-----------|------------|
|{F1633919} | {F1633920} |

This change is a boring follow-up of this one:

https://we.phorge.it/D25507

Test Plan:
Visit the page and click on the new link:

- /config/edit/projects.custom-field-definitions/

Visit this page that was the inspirational page:

- /config/edit/user.custom-field-definitions/

Check that no nuclear implosion is raised.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25552

src/applications/project/config/PhabricatorProjectConfigOptions.php

@@ -105,6 +105,16 @@ EOTEXT
       ),
     );
 
+    $fields_description = $this->deformat(pht(<<<EOTEXT
+List of custom fields for project tags.
+
+For details on adding new fields, see [[ %s | %s ]] in the
+documentation.
+EOTEXT
+      ,
+      PhabricatorEnv::getDoclink('Configuring Custom Fields'),
+      pht('Configuring Custom Fields')));
+
     $subtype_description = $this->deformat(pht(<<<EOTEXT
 Allows you to define project subtypes. For a more detailed description of
 subtype configuration, see @{config:maniphest.subtypes}.
@@ -114,9 +124,7 @@ EOTEXT
     return array(
       $this->newOption('projects.custom-field-definitions', 'wild', array())
         ->setSummary(pht('Custom Projects fields.'))
-        ->setDescription(
-          pht(
-            'Array of custom fields for Projects.'))
+        ->setDescription($fields_description)
         ->addExample(
           '{"mycompany:motto": {"name": "Project Motto", '.
           '"type": "text"}}',