sysctl.9: put negative sense sysctl note in own paragraph

The sysctl man page cautions against negative-sense boolean sysctls
(foobar_disable), but it gets lost at the end of a large paragraph.
Move it to a separate paragraph in an attempt to make it more clear.

This man page could use a more holistic review and edit pass. This
change is simple and straightforward and I hope provides a small but
immediate benefit.

(cherry picked from commit dff1ba09f74725bb0a0b7b8c229ed4e6665b43e6)