Closes #2126 unify_joined: update derive_vars_joined(), derive_var_joined_exist_flag(), filter_joined()

[bundfussr]

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[github-actions]

Package	Line Rate	Health
admiral	98%	✔
Summary	98% (4550 / 4624)	✔

[zdz2101]

small documentation stuff

[zdz2101]

Suggested change

	#' @keywords internal
	#' @keywords internal
	#' @export

Although it will only be used internally, are we not exporting this function at all?

[bundfussr]

I think if we export it, it should be included on the "Reference" page, i.e., it should not be internal.

Do you think it users would like to use it?

[zdz2101]

I doubt our users would use it, but it feels like it has similar "use" as assert_terms() and assert_valid_queries() which are both exported, more for a consistency thing

[bundfussr]

Yes, but it seems that we want to change this (see #2085 (reply in thread)).
@bms63 , what do you think?

[bms63]

Hey @bundfussr - feel free to take or not take any of my suggestions or musings. This one was a bit of epic review!! I think we are good to go once you look through my suggestions.