add notes on CpuUsageInfo

Author: Kaushik Iska

shell/profiling/sampling_profiler.h

@@ -14,8 +14,13 @@
 namespace flutter {
 
 /**
- * @brief
- *
+ * @brief CPU usage stats. `num_threads` is the number of threads owned by the
+ * process. It is to be noted that this is not per shell, there can be multiple
+ * shells within the process. `total_cpu_usage` is the percentage (between [0,
+ * 100]) cpu usage of the application. This is across all the cores, for example
+ * an application using 100% of all the core will report `total_cpu_usage` as
+ * `100`, if it has 100% across 2 cores and 0% across the other cores, embedder
+ * must report `total_cpu_usage` as `50`.
  */
 struct CpuUsageInfo {
   uint32_t num_threads;