[doc](function) Document ARRAY NULL ordering for min max

[Mryange]

What problem does this PR solve?

Issue Number: N/A

Problem Summary: The MIN and MAX documentation incorrectly described NULL array elements as the smallest values, while Doris compares ARRAY values lexicographically with NULL elements greater than non-NULL elements. MIN_BY and MAX_BY accepted ARRAY ordering expressions without documenting this behavior. This update documents the implemented ordering and distinguishes a NULL array value, which is ignored as a NULL aggregate input, from NULL elements inside an ARRAY.

Release note

None

Check List (For Author)

• Test: git diff --check; documentation governance lint could not run because the checkout lacks the gray-matter dependency and yarn is unavailable.
• Behavior changed: No
• Does this need documentation: No