ESQL in-product help update

[abdonpijpelink]

Summary

This PR updates the in-product help for ESQL with the following:

• List COUNT_DISTINCT function under STATS...BY
• Show how to create multi-values with ROW
• Add LENGTH, MV_AVG, MV_MAX, MV_MIN, MV_SUM, and SPLIT functions

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 0e4af2a

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
unifiedSearch	295.7KB	300.4KB	+4.7KB

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[stratoula]

LGTM!