Update distribution documentation about relaxed version restrictions in .make files and best practices

As part of #1371302: Remove restriction on -dev releases in distributions we're going to be much more permissive about what you can use to define versions of stuff in your .make files. So, we're going to want to document:

A) What's now allowed.

B) Why it's a bad idea to use the magic features to find-the-latest-recommended-release. Even though we allow this, it's a bummer since if you checkout a distro from a given release tag and run drush make on the drupal-org.make file, you'll potentially end up with totally different code each time you do it. It's vastly better to have reproducible builds, and that means being explicit in the .make file about what versions of everything you want.