Documentation: sysrq: update description about sysrq crash

After commit 8341f2f2 ("sysrq: Use panic() to force a crash"),
a crash was not generated by dereferencing a NULL pointer anymore.

Let's update documentation as well to make it less misleading.

Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Cc: Matthias Kaehlcke <mka@chromium.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Zefan Li <lizefan.x@bytedance.com>
Link: https://lore.kernel.org/r/20210309191550.3955601-1-hsiangkao@redhat.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>