Fixed #22310 -- Documented exact usage of SECRET_KEY

Thanks to Tim Graham for the review.
Made minor typographic correction to SECRET_KEY docs.

4ad57bbe31bc1813264824111de2f9f74dbda0d6 included a few odd
quotation marks.
Fix formatting error in UUIDField docs.
Fixed comment typo in django/template/__init__.py
Improved AppRegistryNotReady message.
Revert "Improved AppRegistryNotReady message."

This reverts commit 6fa9fa91a5fcb228b3c385b35a2018b3821a447a.

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