Clarify that migrate --check doesn't run migrations

[mlissner]

This very small tweak helps clarify a small difference between the --check flags available to makemigrations and migrate:

• makemigrations --check will make the migrations unless you also add --dry-run.
• migrate --check will not migrate things and doesn't have a --dry-run argument.

I had some existential fear about this difference today when I was using migrate --check as part of an automated deployment pipeline. I assumed, wrongly, that it'd need some kind of --dry-run parameter to prevent it from running migrations (something that's devastating in our big-data environment) and resorted to creating a test to prove myself wrong.

With this change, nobody else will have any doubt in the future.