kqueue.9: Update the description of knlist_clear()

knlist_clear() does not free knotes and so does not call fdrop(), so
remove the bit of the function description which claims otherwise. (The
knote will be dropped by the next queue scan, and it is at that point
that the fd reference will be dropped.)

MFC after: 1 week

(cherry picked from commit 604de40a024c760df5db542171d3953493f56bfb)