Add instructions to Create Repository form fields for Callsign and Short Name

Summary:
When I create new repos, I always forget which is which (between Callsign and Short Name)

This adds helpful text as a placeholder and as control instructions

I am not sure if this layout is 100% better, but this is an option as well

**Add new Repository**
{F3169761 width=800}

**Edit existing Repository**
{F3169775 width=800}

Closes T15742

Test Plan: Added a new repository and saw these control instructions

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15742

Differential Revision: https://we.phorge.it/D25898

src/applications/diffusion/editor/DiffusionRepositoryEditEngine.php

@@ -287,6 +287,9 @@ final class DiffusionRepositoryEditEngine
       id(new PhabricatorTextEditField())
         ->setKey('callsign')
         ->setLabel(pht('Callsign'))
+        ->setControlInstructions(
+          pht('Callsign is the monogram rXXXX for this repository. '.
+          'It cannot contain spaces.'))
         ->setTransactionType(
           PhabricatorRepositoryCallsignTransaction::TRANSACTIONTYPE)
         ->setDescription(pht('The repository callsign.'))
@@ -296,6 +299,9 @@ final class DiffusionRepositoryEditEngine
       id(new PhabricatorTextEditField())
         ->setKey('shortName')
         ->setLabel(pht('Short Name'))
+        ->setControlInstructions(
+          pht('Short Name is part of the URI paths for this repository. '.
+            'It cannot contain spaces.'))
         ->setTransactionType(
           PhabricatorRepositorySlugTransaction::TRANSACTIONTYPE)
         ->setDescription(pht('Short, unique repository name.'))