perf data: Document directory format header: HEADER_DIR_FORMAT

perf data: Document directory format header: HEADER_DIR_FORMAT

We forgot to update the perf.data file format document for the
HEADER_DIR_FORMAT 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: 258031c017c3 ("perf header: Add DIR_FORMAT feature to describe directory data")
Link: https://lkml.kernel.org/n/tip-jbrzb7ijb5al33gi8br6f9rr@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>