MFC r345887:

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

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