docs: Add documentation for TRACE_EVENT_{BEGIN,END}

Bug: https://github.com/google/perfetto/issues/39
Change-Id: I8b093b082a557b4b279e44ed78b44d446840f19c
diff --git a/docs/instrumentation/tracing-sdk.md b/docs/instrumentation/tracing-sdk.md
index f4e6bfd..049388b 100644
--- a/docs/instrumentation/tracing-sdk.md
+++ b/docs/instrumentation/tracing-sdk.md
@@ -92,7 +92,7 @@
 they take care of a number of subtleties (e.g., thread safety, flushing, string
 interning).
 Track events are time bounded events (e.g., slices, counter) based on simple
-annotation tags in the codebase, like this:
+`TRACE_EVENT` annotation tags in the codebase, like this:
 
 ```c++
 #include <perfetto.h>