top: Sync usage and synopsis

Switching between io and cpu sorting uses a great example in the usage.
[-m io | cpu]. Use that everywhere.

MFC after: 3 days
Reviewed by: mhorne
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D49515

(cherry picked from commit 187d954eab94fdcb33609d91966dbd727acfd720)