Explain how and when to use indirect fixtures

[SaturnFromTitan]

As mentioned in #32963 (comment), it would be worthwhile to document this. I'm now sure if we already have consensus on that though. Would be good to make a proposal in a PR and then have a discussion on it.

[mroeschke]

Looks like there hasn't been much activity here in a while.

Additionally, I think a user could just look this up in the pytest docs: https://docs.pytest.org/en/6.2.x/example/parametrize.html#indirect-parametrization

Going to close due to inactivity, but happy to reopen if there's more interest.