Improves varPop and varSamp docs. Adds ...Stable alternative function docs.

[johnnymatthews]

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• Documentation (changelog entry is not required)

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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Closes ClickHouse/clickhouse-docs#2036.

[robot-clickhouse]

This is an automated comment for commit ff97652 with description of existing statuses. It's updated for the latest CI running

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Successful checks

Check name	Description	Status
ClickBench	Runs [ClickBench](https://github.com/ClickHouse/ClickBench/) with instant-attach table	✅ success
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[kssenii]

This PR broke docs check
https://s3.amazonaws.com/clickhouse-test-reports/60024/b538e94acf484f6ced49ce1a135f7db8665becad/docs_check/run.log

Exhaustive list of all broken links found:

- On source page path = /docs/en/sql-reference/aggregate-functions/reference/varpop:
   -> linking to ../syntax#syntax-string-literal (resolved as: /docs/en/sql-reference/aggregate-functions/syntax)
   -> linking to ../syntax#syntax-expressions (resolved as: /docs/en/sql-reference/aggregate-functions/syntax)

- On source page path = /docs/en/sql-reference/aggregate-functions/reference/varsamp:
   -> linking to ../syntax#syntax-expressions (resolved as: /docs/en/sql-reference/aggregate-functions/syntax)