Add documentation for Multithreaded modes

[kgrz]

Issue: #941
Discussion: #937

Although threading depends on the underlying Rack handler, the absence
of any mention of multi-threaded mode/concurrency seems to confuse users
and it was decided to add some pointers to clarify this.

[kgrz]

I probably should add a checklist for other languages. OR should I just use Google Translate?

[davydovanton]

@kgrz I can translate this to russian 😃
P.S.: I think checklist is more suitable here

[kytrinyx]

Yeah, a checklist would be great. I can translate it to French.

[kgrz]

Done, added the checklist on the issue

[DyegoCosta]

@kgrz The Multi-threading section is missing a link on the Table of Content.
And is it intentionally inside the Command Line section?

And I'll translate it to pt-br :D

[kgrz]

@DyegoCosta Good catch! I've added the link under ToC.

Yes, the location under Command Line section was intentional since that was the part that talked about how to start the app. I can move it to a better place if you think there's one.

[DyegoCosta]

@kgrz I was just wondering. Since right now it is all about how to start the app I believe it's ok to be in the Command Line section. Apparently it won't get any bigger but if it does maybe it could have it own section 😅

[kytrinyx]

Thanks!