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