added executor_threads documentation

[111andre111]

Thanks for contributing to Logstash! If you haven't already signed our CLA, here's a handy link: https://www.elastic.co/contributor-agreement/

[111andre111]

Here was a parallel one:
#409

[andsel]

Just left a minimal suggestion

[111andre111]

@andsel
What is the influcence of the draft PR #410
mentioned in #409?

[andsel]

that description is more extended, we could use that, and close that draft PR with this one

[andsel]

the #410 is some more deep, where the #409 and this are only a face of the problem it's trying to fix

[andsel]

Hi @111andre111 I've rebased to master branch fixing conflicts and improved description.

[111andre111]

So I assume we can merge this one and #409 as well if there weren't duplicates plus I assume #410 still will need some investigation time, correct? @andsel

[andsel]

Yes correct @111andre111, but Changelog needs a fix

[111andre111]

@andsel I have fixed now the Changelog

[karenzone]

Thank you for adding this important documentation. Nice job explaining it. I left some minor suggestions inline for your consideration.

[andsel]

LGTM, merge squashing the commits please, or ping me if you need I do it

[111andre111]

@andsel Sorry if I didn't manage the squash. I always got an error message.

[andsel]

@111andre111 I've rebased to master there were some conflicts on CHANGELOG.md ao probably that prohibited the merge.
I don't publish yet this, I left the version x.y.z so that a next bugfix or feature also publish this.

[111andre111]

Ah I see. I didn't see that. Thank you @andsel