Revisit/Update how we explain Highest Imputation (vignette and function documentation)

[bms63]

This comes out of the #2028 where the imputation vignette has discussion stating that admiral does not allow a user to preserve parts of the partial dates. This is not true and we decided to remove this section from the vignette.

In the PR to address this issue a poor attempt was made to better explain highest imputation. I found working through the examples to be the best way to understand H.I., but trying to explain it in a pithy and concise way was a bit of a struggle. I do believe a user can grasp the H.I. and the date/time functions by working from the examples as they are scaffold really well...but I think the documentation is hard to follow.

I think we should discuss this issue here and in upcoming meetings and then update how we explain H.I. I was leaning towards a diagram/flowchart as the max/min, date/time and preserve arguments all forms part of the discussion around H.I.!

Also...pointed about by @zdz2101 that in every release we tend to have a bug around out dtm/dt functions

Some relevant links:

#2088 (comment)
https://pharmaverse.github.io/admiral/cran-release/reference/derive_vars_dtm.html

@pharmaverse/admiral

[manciniedoardo]

Discussed - we shall close as no user feedback that this is required.