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: cf790516 ("perf record: Encode -k clockid frequency into Perf trace")
Link: https://lkml.kernel.org/n/tip-slhnjp06027j3ae17qqetzxj@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf.data-file-format.txt

@@ -296,6 +296,12 @@ $ perf report --header-only -I
 # memory nodes (nr 1, block size 0x8000000):
 #    0 [7G]: 0-23,32-69
 
+	HEADER_CLOCKID = 23,
+
+One uint64_t for the clockid frequency, specified, for instance, via 'perf
+record -k' (see clock_gettime()), to enable timestamps derived metrics
+conversion into wall clock time on the reporting stage.
+
         HEADER_BPF_PROG_INFO = 25,
 
 struct bpf_prog_info_linear, which contains detailed information about