perf data: Add description of header HEADER_BPF_PROG_INFO and HEADER_BPF_BTF

perf data: Add description of header HEADER_BPF_PROG_INFO and HEADER_BPF_BTF

This patch addes description of HEADER_BPF_PROG_INFO and HEADER_BPF_BTF to
perf.data-file-format.txt.

Requested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: 606f972b1361 ("perf bpf: Save bpf_prog_info information as headers to perf.data")
Link: http://lkml.kernel.org/r/20190521064406.2498925-1-songliubraving@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>