bootstat: clarify unit.

Test: pass
Change-Id: I7fc5cf9a5ae8ff895e20922bc7266ef210e8659f
diff --git a/bootstat/bootstat.cpp b/bootstat/bootstat.cpp
index 8979e9a..ee1ae31 100644
--- a/bootstat/bootstat.cpp
+++ b/bootstat/bootstat.cpp
@@ -1322,6 +1322,8 @@
 
   // Record the total time from device startup to boot complete, regardless of
   // encryption state.
+  // Note: we are recording seconds here even though the field in statsd atom specifies
+  // milliseconds.
   boot_event_store.AddBootEventWithValue(boot_complete_prefix, uptime_s.count());
 
   RecordInitBootTimeProp(&boot_event_store, "ro.boottime.init");