Add a link to the definitions of release types to the Drupal core release customizations

It's hard to tell from the feature source, but based on the fact that the existing string seems to appear only on the core node and not on a couple other projects I selected at random, I think this will change only the core project node.

Text looks fine. For reference it would appear once at this place, where I drew the jiggly lines.

Deployed!

Thanks both!