bus_generic_print_child.9: Document bus_print_domain

While here, cross-reference BUS_PRINT_CHILD (and fix a stale reference
to DEVICE_PRINT_CHILD which doesn't exist) and expand the text
describing the role of the helper functions.

Differential Revision: https://reviews.freebsd.org/D48373

(cherry picked from commit 36ecb25e6158f16356938432defd9f9df67a1366)