Turn the return_value and not_a_test_method health checks into strict errors

[Zac-HD]

This is pretty much just code cleanup: unlike the other healthchecks which are basically performance checks, I'm pretty sure there's never a situation where disabling these allows you to do something useful. I think we should therefore:

• Turn the return_value and not_a_test_method health checks into strict errors.
• Define HealthCheck.__iter__() and .all() such that they're omitted from that
• Use note_deprecation() if the attributes are accessed on HealthCheck
• Merge the healthcheck docs into the main settings docs
• [me, after we merge] add a redirect on readthedocs to avoid 404'ing links