Bring old repository instructions and guidance forward to new UI

Summary:
Ref T10923. Fixes T10406. This brings most of the guidance/instructions forward:

  - Some remained as instructions.
  - Some moved to documentation.

Test Plan: Went through all of the sections and hit the help.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10406, T10923

Differential Revision: https://secure.phabricator.com/D15873

src/applications/diffusion/controller/DiffusionRepositoryEditActivateController.php

@@ -40,7 +40,10 @@ final class DiffusionRepositoryEditActivateController
 
     if ($repository->isTracked()) {
       $title = pht('Deactivate Repository');
-      $body = pht('Deactivate this repository?');
+      $body = pht(
+        'If you deactivate this repository, it will no longer be updated. '.
+        'Observation and mirroring will cease, and pushing and pulling will '.
+        'be disabled. You can reactivate the repository later.');
       $submit = pht('Deactivate Repository');
     } else {
       $title = pht('Activate Repository');