doc: Explain NULLs handling for `array_*` builtin functions

[comphead]

This reminds me of our earlier discussion about arrays with NULL in them. Should we document at a high level somewhere what semantics we're adopting arrays with NULL in them?

Originally posted by @mbutrovich in #21013 (review)

The idea to add NULL handling behavior to description, for example:

#[user_doc(
    doc_section(label = "Array Functions"),
    description = "Removes the first `max` elements from the array equal to the given value. <ADD HERE expectation when dealing with NULLs>",
    syntax_example = "array_remove_n(array, element, max))",

[Xavrir]

Hello!, I want to help with this issue

[Xavrir]

Opened #21018 to clarify the documented NULL behavior for , , and , and to add regression coverage for scalar and row-wise NULL inputs.

[Xavrir]

Correction: opened #21018 to clarify the documented NULL behavior for array_remove, array_remove_n, and array_remove_all, and to add regression coverage for scalar and row-wise NULL inputs.