Clarify `requires-nightly` in issue triage chapter

[Noratrieb]

I wonder if we should expand upon this. For example, personally speaking I only add this label if it's a nightly-to-nightly regression (i.e., a super recent regression) but typically not if I also add F-* labels since that implies nightly in most cases.

Originally posted by @fmease in #716 (comment)

[fmease]

Should all A-rustdoc-json/A-cranelift issues also have requires-nightly?

Originally posted by @aDotInTheVoid in #716 (comment)

[Noratrieb]

I don't know what it should be about exactly. I think requires-nightly is most useful in combination with, say I-unsound. I-unsound requires-nightly makes it clear that hey, this issue is bad, not but affecting stable, so everything is fine for now.