doc: highlight that doc counts come from lucene

[thefourtheye]

The docs don't clearly explain that the deleted doc count also comes from lucene.
IMHO, it is worth highlighting this information separately, as a Note.
Apart from that, there should be an official recommended alternative as well.

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?
• If submitting code, have you built your formula locally prior to submission with gradle check?
• If submitting code, is your pull request against master? Unless there is a good reason otherwise, we prefer pull requests against master and will backport as needed.
• If submitting code, have you checked that your submission is for an OS that we support?

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[thefourtheye]

Bump!

[thefourtheye]

Bump!

[nik9000]

Sorry this sat for so long! The change makes sense to me and renders well. I'll merge and cherry-pick as appropriate.

[nik9000]

Merged and cherry-picked to 5.3, 5.4, and 5.x branches. Thanks @thefourtheye!

[thefourtheye]

Thanks @nik9000 :-)