crash.8: update a little

Replace "cannot mount root" (which is no longer a panic) with a brief
description of the mountroot> prompt, and recommend using "system
install media" rather than the fixit floppy.

Remove "timeout table full" entirely since this panic no longer exists.

MFC after: 3 days
Reviewed by: ziaee
Differential Revision: https://reviews.freebsd.org/D51512

(cherry picked from commit c9868af774ec24bcf6914fc971378e9723eb44ab)