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: 606f972b13 ("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>

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

@@ -272,6 +272,22 @@ struct {
 
 Two uint64_t for the time of first sample and the time of last sample.
 
+        HEADER_BPF_PROG_INFO = 25,
+
+struct bpf_prog_info_linear, which contains detailed information about
+a BPF program, including type, id, tag, jited/xlated instructions, etc.
+
+        HEADER_BPF_BTF = 26,
+
+Contains BPF Type Format (BTF). For more information about BTF, please
+refer to Documentation/bpf/btf.rst.
+
+struct {
+	u32	id;
+	u32	data_size;
+	char	data[];
+};
+
         HEADER_COMPRESSED = 27,
 
 struct {