Pr-Guidelines Article: make it clear that the article is Bugzilla-specific

[linimon]

Summary

This article has not been updated in years. There is so much wrong, that it will need to be tackled in multiple patches.

This first patch simply states that the article is Bugzilla-specific, and that any use of "PR" means "Bugzilla Problem Report". In a few cases, this is now spelled out explicitly.

[lwhsu]

I think expending "PR" to "problem report" is a good thing, as nowadays that "PR" might be known by more people as "pull request"
While I don't think we'll be moving away from Bugzilla anytime soon, I suggest that keeping the title of this article independent of the issue tracking software we're using so it might be easier to update in the future.
We can mention in the beginning of this articles that the current issue tracking software the FreeBSD project uses is Bugzilla and this article will mainly cover it.

[lwhsu]

In D53645#1224816, @lwhsu wrote:

We can mention in the beginning of this articles that the current issue tracking software the FreeBSD project uses is Bugzilla and this article will mainly cover it.

oh I didn't see D53644 while writing this. Then I think it's fine.

[imp]

documentation/content/en/articles/pr-guidelines/_index.adoc

72

What is this 'Needs MFC' state? Usually the bug is marked as 'In Progress' still, with the MFC keywords set to act as a reminder to MFC.

[linimon]

documentation/content/en/articles/pr-guidelines/_index.adoc

72

This is just a hold-over from the existing text.