Document TEST_MIRROR caveat

[christianbundy]

Problem: This setting only works in transactions but that isn't documented.

Solution: Add caveat to documentation.

Link: https://code.djangoproject.com/ticket/23718

[felixxm]

@christianbundy Thanks for this patch 👍 IMO we should also add a note to the "Testing primary/replica configurations" section.

[felixxm]

Closing due to inactivity.

[christianbundy]

I think that this is a net positive, can we merge as-is or is it incomplete without adding a note somewhere else?

[stickperson]

Any reason this has not been merged? I spent a non-trivial amount of time figuring this out today.