docs: add documentation about Trace being the root proto
Change-Id: I1d608372217e5e9a4b03fcc9eab8e239db04449b
diff --git a/docs/reference/synthetic-track-event.md b/docs/reference/synthetic-track-event.md
index 1052d77..0e2c3b9 100644
--- a/docs/reference/synthetic-track-event.md
+++ b/docs/reference/synthetic-track-event.md
@@ -11,6 +11,12 @@
[text format](https://protobuf.dev/reference/protobuf/textformat-spec/)
representation of protobufs.
+The root container of the protobuf-based traces is the
+[Trace](https://cs.android.com/android/platform/superproject/+/master:external/perfetto/protos/perfetto/trace/trace.proto)
+message which itself is simply a repeated field of
+[TracePacket](https://cs.android.com/android/platform/superproject/+/master:external/perfetto/protos/perfetto/trace/trace_packet.proto)
+messages.
+
## Thread-scoped (sync) slices
NOTE: in the legacy JSON tracing format, this section correspond to B/E/I/X
events with the associated M (metadata) events.
