panic(9): some updates

• Better description of what the panic() function does
• Document KERNEL_PANICKED()
• Add a section describing panic execution context
• Add SEE ALSO

Reviewed by: kib, markj, rpokala
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39132

(cherry picked from commit d5e105bf7e4cb7e80eb4a4f8960bd6dd12fc4885)