perf mem: Describe overhead calculation in brief

Unlike perf-report which uses sample period for overhead calculation,
perf-mem overhead is calculated using sample weight. Describe perf-mem
overhead calculation method in it's man page.

Reviewed-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20250523222157.1259998-1-namhyung@kernel.org


Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>