Rewrite intro(4) man page.

Rewrite intro(4) man page.

- Remove issues that no longer apply thanks to devfs
- Add language pointing out devfs's role and referencing its config
- Add a "historical notes" section and move discussion of block vs character devs to it, including pointing out the removal of block devs
- Modernize some examples

MFC after:	1 week
PR:		236970
Submitted by:	andrew@tao173.riddles.org.uk
Reviewed by:	0mp
Differential Revision:	https://reviews.freebsd.org/D19799