Document Diagnostic Port

[noahfalk]

Fixes dotnet/diagnostics#2900

We added the diagnostic port in 3.1 and expanded on it in 5.0, but never
documented it aside from tangential references in some tools that used it.
This change describes it directly and adds some detail that wasn't mentioned
elsewhere. I also tried to improve docs on some related environment variables
that misleadingly claimed scenarios were only for dotnet-monitor or only for
Mono.

[noahfalk]

I'm still need to clean up the lint stuff and build warnings, but it should be close enough to do a decent review
@lateralusX - Can you check if the new content is saying the right things about Mono? Feedback on anything else is welcome too of course.
@dotnet/dotnet-diag

[noahfalk]

Thanks @lateralusX 👍 I think I had a little tunnel vision associating Mono with Android/iOS/tvOS. I pivoted the description to focus on the operating system and hopefully I've got Mono included in all the right places now.

[tdykstra]

Looks good, just some grammar and style suggestions.

[josalem]

LGTM sans a couple nits I commented on 👍

[noahfalk]

Thanks all!
@josalem - two of your comments I was unsure about, otherwise I think this is ready to go.

[noahfalk]

@josalem - got it and yeah I can see the confusion. I'm going to guess that even if people are confused and thought diagnostic port was used to implement debugging they would still conclude that debugging is effected. If you have any thoughts on a better way to say it we could always do a tiny PR that adjusts it : ) Thanks!