Fix archive additional_metadata docs + maybe rename (or remove?)

[brookslogan]

Current docs include

#' @param additional_metadata List of additional metadata to attach to the
#'   `epi_archive` object. The metadata will have `geo_type` and `time_type`
#'   fields; named entries from the passed list or will be included as well.

The archive's $additional_metadata doesn't actually get geo & time type attached, but looks like if there are other_keys that they would be put there. If it does have this extra stuff attached, it should be called metadata instead, similar to epi_df. But do we even need an $additional_metadata? Why not just an $other_keys? If users want to attach more metadata, they can just use attr<-; we're not doing anything special/extra.