[Docs][Elasticsearch] Specify metricsets list behaviour with xpack mode enabled

[tonyghiani]

Based on an offline conversation, when the xpack.enabled: true for the elasticsearch module, the documentation didn't clearly state the expected metricset list when the metricsets property is still set to some entries.

This update should clarify what happens when both xpack.enabled: true and metricsets are configured.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @tonyghiani? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[jennypavlova]

LGTM! Thanks for adding this!