Documentation: Better document the softlockup_panic sysctl

Documentation: Better document the softlockup_panic sysctl

Commit 607a2f2ec14b ("softlockup: allow panic on lockup") added the
softlockup_panic sysctl, but didn't add information about it to the file
Documentation/admin-guide/sysctl/kernel.rst (which in that time certainly
wasn't rst and had other name!).

This patch just adds the respective documentation and references it from
the corresponding entry in Documentation/admin-guide/kernel-parameters.txt.

This patch was strongly based on Scott Wood's commit 6eed36811a2b
("Documentation: Better document the hardlockup_panic sysctl").

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Link: https://lore.kernel.org/r/20200310183649.23163-1-gpiccoli@canonical.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>