[add_docker_metadata] update docs

[VihasMakwana]

This PR updates documentation of add_docker_metadata processor.
The parameter hostfs was not documented previously.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[github-actions]

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Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @VihasMakwana? 🙏.
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-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/auditbeat/add-docker-metadata.md
• docs/reference/filebeat/add-docker-metadata.md
• docs/reference/heartbeat/add-docker-metadata.md
• docs/reference/metricbeat/add-docker-metadata.md
• docs/reference/winlogbeat/add-docker-metadata.md

[colleenmcginnis]

This language might be clearer if it still aligns with what hostfs does:

Suggested change

	: (Optional) Specifies the mount point of the host’s filesystem for use in monitoring a host from within a container.
	: (Optional) Specifies the mount point of the host’s filesystem, which can be used to monitor a host from within a container.