aes: xts: Add a reference to the standard
It's useful to add a reference to the standard one implements. Add a
reference to the AES-XTS standard to the top of `aes.h`.
* <em>ISO/IEC 18033-2:2006: Information technology -- Security
* techniques -- Encryption algorithms -- Part 2: Asymmetric
* ciphers</em>.
+ *
+ * The AES-XTS block mode is standardized by NIST SP 800-38E
+ * <https://nvlpubs.nist.gov/nistpubs/legacy/sp/nistspecialpublication800-38e.pdf>
+ * and described in detail by IEEE P1619
+ * <https://ieeexplore.ieee.org/servlet/opac?punumber=4375278>.
*/
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