As stated in the previous commit, option '-J' was introduced by commit
"Add -J to filter by matching jail IDs and names."
(13767130c7147ae7182a, r265229), which unfortunately talked about '-J'
being a filter while actually implementing it as a regular selection
option which adds to the processes to display.

The manual page's formulation hinted more at '-J' being a filter, which
it is not, or could be just considered ambiguous, because of the
presence of the "only" word. Consequently, remove it and reformulate.