Problem-Reports article: modify to indicate which parts are Bugzilla-specific

[linimon]

Summary

This Article was written long before Phabricator and Pull Requests were accepted ways to submit change requests or bug issues to FreeBSD.

As a first step to bringing it up to date, disambiguate the "PR" term, and spell it out as "Bugzilla Problem Request".

There are many more problems here to be solved.

[lwhsu]

documentation/content/en/articles/problem-reports/_index.adoc

54

I would like to say something like:

"to submitting patches via Phabricator review system" or pull requests to the repositories in the FreeBSD GitHub organization"

133

we still have one "PR" here, intentional?

173

another remaining "PR", also intentional?

274

I guess it's last "PR" here.

[imp]

Generally I like it. I generally agree with lwhsu's comments too, but will hit 'ship it' with the proviso you make him happy.