perlrun/debguts: Connect -dt and PERL5DB_THREADS a bit better

[tonycoz]

This was going to cover more, but it turned out to already be fairly well covered.

Fixes #22854

Fix a documentation nit

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• This set of changes does not require a perldelta entry.

[khwilliamson]

lgtm,

@KES777 is this sufficient?

[KES777]

so when perl see -dt option it will just set PERL5DB_THREADED=1. It does nothing to guess what is threading backend will be used in the script, right?

If yes, probably it would be nice to mention on perldebguts that:

Perl knows nothing about threading used inside debugged script and can not check that.

Otherwise we are good to go.