perf data: Document clockid header: HEADER_CLOCKID

perf data: Document clockid header: HEADER_CLOCKID

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

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.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: Peter Zijlstra <peterz@infradead.org>
Cc: Simon Que <sque@chromium.org>
Fixes: cf7905165fee ("perf record: Encode -k clockid frequency into Perf trace")
Link: https://lkml.kernel.org/n/tip-slhnjp06027j3ae17qqetzxj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>