add docs for aggregation function datasketches_hll_union_agg - #3711

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English 4.x and current docs are strongly synchronized%2C but the current counterpart is missing. Add it or explain the version-specific exception in the PR description. Owner%3A @zclllyybb

Every fenced sql query in Example must be followed by a fenced text block with expected output. Owner%3A @zclllyybb

Return Value section must document NULL return conditions%2C including when the function never returns NULL. Owner%3A @zclllyybb

Parameters section must be a Markdown table with Parameter and Description columns. Owner%3A @zclllyybb

Parameters section must be a Markdown table with Parameter and Description columns. Owner%3A @zclllyybb

SQL function docs must contain sections in this order%3A Description%2C Syntax%2C Parameters%2C Return Value%2C Example. Missing%3A Example. Owner%3A @zclllyybb

Rendered SEO title is duplicated across indexable pages%3A "DATASKETCHES_HLL_UNION_AGG - Apache Doris". Add a version%2C locale%2C or page-specific qualifier. Owner%3A @zclllyybb

Rendered SEO title is duplicated across indexable pages%3A "DATASKETCHES_HLL_UNION_AGG - Apache Doris". Add a version%2C locale%2C or page-specific qualifier. Owner%3A @zclllyybb

SEO description should be 80-160 characters; current length is 189. Owner%3A @zclllyybb