Correct and clarify inline docs for excluded_terms in get_adjacent_post()

[grapplerulrich]

The variable $excluded_terms is an array of excluded term IDs. Empty string if none were provided.

It is documented incorrectly in ​`get_{$adjacent}_post_where` and ​`get_{$adjacent}_post_join`.

Correctly document $excluded_terms is an array of excluded term IDs or an empty string is none provided.

Trac ticket: https://core.trac.wordpress.org/ticket/56348

[costdev]

Thanks @grapplerulrich, and nicely spotted! $excluded_terms is indeed documented as int[]|string everywhere but in these two locations.

​PR 3073 Looks good to me! Milestoning for 6.1.

[ocean90]

In 53870:

Docs: Consistently document the $excluded_terms variable in get_adjacent_post().

The variable $excluded_terms is an array of excluded term IDs or empty string if none were provided.

Props grapplerulrich, costdev.
Fixes #56348.