kern_reboot(9): some updates

• This function no longer disables interrupts
• MLINK to reboot.9
• The mentions of autoconfiguration is more about shutdown_nice(), coming in the next commit.
• Describe the RB_* flags relevant to this function
• Describe behaviour when shutdown hooks fail the reset
• Describe expected execution contexts
• Add FF copyright
• xref panic(9)
• xref this page in reboot(2)

Reviewed by: markj
Discussed with: rpokala, Pau Amma <pauamma@gundo.com>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39133