Document the differences for column re-ordering in `new_epi_df` and `as_epi_df`

It probably would be good to document the differences for column re-ordering in`new_epi_df` and `as_epi_df`. For ex., `new_epi_df` always has column re-ordering so that the columns are `geo_value, time_value ...` . In contrast, `as_epi_df` does not appear to enforce this column re-ordering when we're already feeding an epi_df to it (it simply returns the `epi_df` object unchanged). But it does enforce the column re-ordering if we are converting a `tbl_df`, `data frame`, or `tbl_ts` to an `epi_df` (as it ultimately calls `new_epi_df`).

Locate the documentation and entry points for `new_epi_df` and `as_epi_df`, then compare how each handles inputs that are already `epi_df` objects versus data frames or tibbles. Done means the documentation clearly states when columns are reordered and when an existing `epi_df` is returned unchanged.

Written by the indexing model from the issue text.