DEVICE_IDENTIFY.9: Modernize description and use cases

Mention adding devices based on firmware tables and software-only
pseudo-devices as use cases for identify methods as those are more
common than reading random I/O ports to identify a legacy ISA device.

Describe how device_find_chid can be used to avoid duplicates. While
here, explicitly note that devices added in identify methods typically
use a fixed device name.

Trim the cross-references a bit.

Reviewed by: ziaee, imp
Differential Revision: https://reviews.freebsd.org/D48367