stty(1): provide details about interaction with job control

Describe a shell trick to do non-blocking modification of the terminal
settings, by ignoring job control signals with trap built-in.

PR: 266627
With input from: jilles
Reviewed by: pauamma
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D36745