Clarify return value in device auth fn comment (youtube#3849)

b/341795489

cobalt/browser/device_authentication.h

@@ -24,7 +24,10 @@ namespace cobalt {
 namespace browser {
 
 // Returns a base64 encoded SHA256 hash representing the device authentication
-// signature based on the device certification scope and the current time.
+// signature, the device certification scope, and the current time as query
+// parameters in a string. The signature is the result of signing a message,
+// containing the certification scope and the current time, with a secret key
+// provided by the device.
 std::string GetDeviceAuthenticationSignedURLQueryString();
 
 // The following methods are deterministic helper functions used in the