`?epi_df` doesn't pull up help (any more?)

[brookslogan]

[Related:] From a failing check in epipredict:

* checking Rd cross-references ... WARNING
Warning: Missing link or links in Rd file 'get_test_data.Rd':
  ‘epi_df’

[plus documenting epipredict gives things like

✖ arx_forecaster.R:3: @description refers to unavailable topic epiprocess::epi_df.

]
Seems likely that users might also try ?epi_df, so we should make that work (again?).

[dshemetov]

This is probably something that got adjusted in #472. I'll take a look.