perf kmem: Document a missing option & an argument

First, 'perf kmem' has a '--force' option, but didn't document it on the
man page. So add it.

Second, the '--time' option has to get a value, but isn't documented on
the man page. Describe it.

Signed-off-by: Sangwon Hong <qpakzk@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Taeung Song <treeze.taeung@gmail.com>
Link: http://lkml.kernel.org/r/1518381517-30766-1-git-send-email-qpakzk@gmail.com


[ Add blank like after --force block, as requested by Namhyung ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>