docs: 📝 expand on inclusions and exclusions

[Aastedet]

ChatGPT seems to understand what I'm trying to describe (although it shouldn't apply the exclusion functions just yet): https://chatgpt.com/share/66eab6f4-0ffc-800a-aedb-d9dae21c48c3 😄

Closes #130
Closes #140

[Aastedet]

Since the join_lpr2() and join_lpr3() functions already perform variable selection, we should also include a baseline filtering operation, since that is what we would do in practice to avoid processing unnecessary data: load-> select variables -> filter to only diagnoses relevant to the osdc package (similar to how the include_gld_purchases() function filters to A10*).

Can we do this as-is (my uneducation opinion), or do we need to change the function name?

[lwjohnst86]

@Aastedet it sounds like we need some processing before joining then. Could you document those filtering steps in the function flow/design docs?

[lwjohnst86]

This is coming along nicely!! Just some comments for now.

[Aastedet]

@Aastedet it sounds like we need some processing before joining then. Could you document those filtering steps in the function flow/design docs?

Yes - I've added them in the latest commit