Update comments around PLATFORM_SECURITY_LEVEL
am: 65994c1d13

Change-Id: I800ffe93f311d9871c525f537cb92c525ac51510
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 41f76ba..1505312 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -65,7 +65,9 @@
 
 ifeq "" "$(PLATFORM_SECURITY_PATCH)"
   # Used to indicate the security patch that has been applied to the device.
-  # Can be an arbitrary string, but must be a single word.
+  # It must signify that the build includes all security patches issued up through the designated Android Public Security Bulletin.
+  # It must be of the form "YYYY-MM-DD" on production devices.
+  # It must match one of the Android Security Patch Level strings of the Public Security Bulletins.
   #
   # If there is no $PLATFORM_SECURITY_PATCH set, keep it empty.
   PLATFORM_SECURITY_PATCH := 2016-10-05