Update hints autodiscovery documentation

[tetianakravchenko]

Align documentation with changes in PR - elastic/elastic-agent#4625

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[mergify]

This pull request does not have a backport label. Could you fix it @tetianakravchenko? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[constanca-m]

I see other references to /etc/elastic-agent/inputs.d in the documentation. With the change of volume, they stop being correct, right?

[tetianakravchenko]

I see other references to /etc/elastic-agent/inputs.d in the documentation. With the change of volume, they stop being correct, right?

@constanca-m thanks! done 3df38c0

[kilfoyle]

LGTM! 🚀

[kilfoyle]

LGTM (on behalf of Platform-Docs). 🚀

[tetianakravchenko]

@kilfoyle could you please merge this PR? I don't have write access to this repo