Clarify that the -c argument clears the list of tracepoints specified by
-t (it does not clear all tracepoints).

Submitted by:	jmg, Eric van Gyzen <eric@vangyzen.net>
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270674 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f