Improved documentation / UX for interaction terms.

[pitakakariki]

Scan https://github.com/pitakakariki/simr/issues/91 for potentially confusing options.

[pitakakariki]

From: https://github.com/pitakakariki/simr/issues/99#issuecomment-1061402313

"I'm posting this for anyone else troubleshooting power analysis in simr for interaction effects: if you're getting power for test="t" but not "chisq", "anova", or "f", or vice versa, check how you're specifying the interaction term in the test function. For the "t" test the interaction effect should appear as it looks in summary(yourmodel). For the other tests, it should appear as it looks in Anova(yourmodel). This tripped me up quite a bit, hope it helps someone else!"