Manifest: fix error message about ~alias

Context

An error message was not updated when we changed ~runtest into ~alias.

I took this opportunity to warn in the documentation too.

Manually testing the MR

Declare a test with ~opam:"". It should fail since the default ~alias is "runtest", i.e. non-empty.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR