docs: Align collections NULL-argument handling with compatibility layer

[imilinovic]

Release note

Documented NULL-argument and NULL-element handling for the collections.* functions and procedures, including the per-function result table.

Related product PRs

PRs from product repo this doc page is related to:
memgraph/memgraph#4416

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Project	Deployment	Actions	Updated (UTC)
documentation	Ready	Preview, Comment	Jul 20, 2026 1:37pm

[imilinovic]

unified in #1694