Reapply: "bus.h: rewrite comment describing intr_type"

Describe more clearly the purpose of these flags and where they are
used.

Remove grog's comment about revisiting the flags. At this point it does
not make sense to change this long-standing API, especially in light of
2cf7870864ea ("Collapse interrupt thread priorities").

Give a better example of a bus modifying these flags; I did not find any
existing instance where a bus changes the interrupt's type.

Reviewed by: imp, markj, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36896

Note: 2cf7870864ea is not present in stable/13 and likely will never be.
While reapplying this change, I have modified the comment further to
eliminate any inaccuracy Re: priorities. It should be applicable even if
the change is merged some day.

(cherry picked from commit af390486010e8fc78c4d715e751cd1175d0752c7)