Enhance JCA Reference Guide with GCM cipher examples

AES cipher with GCM mode is an AEAD cipher which has somewhat different usage patterns than the non-AEAD ciphers. Besides the regular data for encryption/decryption, it also takes AAD which is optional, but when present, must be supplied before the data for encryption/decryption is fed. In addition, in order to use GCM securely, callers should not re-use key+iv combinations for encryption. This means that the cipher object should be explicitly re-initialized with a different set of parameters every time for each encryption operation. The javadoc for javax.crypto.Cipher is enhanced to mention these. But, it'd be helpful to update JCA reference guide with more details and sample code to get the points through.