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