Ensure doc consistency about explicit/implicit versions in epi_archive

[brookslogan]

Currently, some core operations of epi_archive rely on "implicit" versions formed by using the last version of each observation carried forward, rather than an "explicit" listing of every row in every version that we should count as observed. (A user can provide an explicit representation, with some caveats: we will be interpolating between their versions, and they have to use a workaround to represent row deletions.) We are also changing epix_merge from treating the implicit treatment from optional to mandatory. We should make sure the docs reflect this. Checking over docs on main, it looks like we might not be fully consistent about this in the roxygen docs:

• epi_archive is mixed: DT version column description sounds like explicit, but LOCF paragraph describes implicit.
• as_of describes implicit
• epix_slide description is compatible with either, assuming we only allow one.
• current (pre-Km compactify rectify #101) epix_merge docs confuse the issue, as they describes implicit-like behavior as optional

The vignettes should also be checked to make sure they are accurate.