Document that internal functions follow the `action.py` with `object()` style

[lwjohnst86]

As per seedcase-project/seedcase-sprout#263 discussion, do this so we can do something like:

import verify

verify.is_file()

[lwjohnst86]

In practice, using the internals/ folder with from PACKAGE.internals import _FUNCTION
seems to work pretty well. So will close this.