perf kmem: Improve man page for record options

Since:

  https://lore.kernel.org/lkml/20200708183919.4141023-1-irogers@google.com/

The output option works for 'perf kmem', however, it must appear after
'record'. This is different to 'stat' where '-i' for the input must
appear before. Try to capture this complication in the man page.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210922212031.485950-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>