[1.7.x] Fixed #22310 -- Documented exact usage of SECRET_KEY

Backport of 4ad57bbe31bc1813264824111de2f9f74dbda0d6 from master.
[1.7.x] Made minor typographic correction to SECRET_KEY docs.

Backport of 2bb00b0b0b5232aff806c8a9b3f1126c1cf79221 from master.
[1.7.x] Improved AppRegistryNotReady message.

Backport of 6fa9fa91a5 from master
[1.7.x] Revert "Improved AppRegistryNotReady message."

This reverts commit 154f5f0de108f428de2d5f488e0264f4459a4b66.

Aymeric: "I chose not to talk about django.setup() here on purpose.
This will hardly always be the correct solution."

Backport of 8121860be4 from master