docs: clarify NULL handling for array_remove functions (#21014)

[Xavrir]

Summary

• clarify how array_remove, array_remove_n, and array_remove_all behave when the input array contains NULL values
• document that a NULL element argument returns NULL instead of removing NULL entries from the array
• add examples that show existing NULL array elements are preserved when removing a non-NULL value

Testing

• docs-only change, no code paths changed

Closes #21014

[comphead]

Thanks @Xavrir I'd suggest to leave only documentation updates, I think the test attached duplicating what we already have in the codebase

[Xavrir]

Thanks for pointing that out. I trimmed the duplicated tests and kept the PR focused on the documentation update in 38fa468.