perf data: Document memory topology header: HEADER_MEM_TOPOLOGY

We forgot to update the perf.data file format document for the
HEADER_MEM_TOPOLOGY header, do it now from comments in the patch
introducing it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Chong Jiang <chongjiang@chromium.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Simon Que <sque@chromium.org>
Fixes: e2091ced ("perf tools: Add MEM_TOPOLOGY feature to perf data file")
Link: https://lkml.kernel.org/n/tip-h5lcm1nbe9ztxwm61gmadd56@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>