kprobetrace, doc: Add the explanation to remove probe points

Latest kprobetrace can remove probe points selectively, thus
the documentation should be updated too.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: systemtap <systemtap@sources.redhat.com>
Cc: DLE <dle-develop@lists.sourceforge.net>
LKML-Reference: <20100119023512.31880.35535.stgit@localhost6.localdomain6>
Signed-off-by: Ingo Molnar <mingo@elte.hu>