Describe accepted return types/classes of `f` in `epi[x]_slide` roxygen

[brookslogan]

There are constraints on the functions f that we accept in the slide functions. We need to describe this in the docs.

The code is hard to parse and hard to say what is actually accepted in f.

[dshemetov]

@nmdefries @brookslogan any suggestions for how to tackle this? Docs in question.

[brookslogan]

I'm guessing this will involve some amount of rooting through source code and testing. And some may be in flux with epi_slide() work.

Check:

• For tidyeval way: can we output vectors? tibbles? lists of tibbles or other things?
• For non-tidyeval way: same questions
• What lengths/nrows can we output? (epi_slide has more demands here; either 1 or the number it's thinking of; epix_slide shouldn't)