Warn about certificate serial number size

Bug: 343627215
Test: None, comment change only
Change-Id: I362e30991ca951670ae29e9a1d9b51cf31c2ef71

keystore/java/android/security/keystore/KeyGenParameterSpec.java

@@ -1149,6 +1149,8 @@ public final class KeyGenParameterSpec implements AlgorithmParameterSpec, UserAu
 
         /**
          * Sets the serial number used for the certificate of the generated key pair.
+         * To ensure compatibility with devices and certificate parsers, the value
+         * should be 20 bytes or shorter (see RFC 5280 section 4.1.2.2).
          *
          * <p>By default, the serial number is {@code 1}.
          */