xtea: Clarify that the current API works in big endian mode

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/xtea.h

@@ -49,12 +49,14 @@ AVXTEA *av_xtea_alloc(void);
  * Initialize an AVXTEA context.
  *
  * @param ctx an AVXTEA context
- * @param key a key of 16 bytes used for encryption/decryption
+ * @param key a key of 16 bytes used for encryption/decryption,
+ *            interpreted as big endian 32 bit numbers
  */
 void av_xtea_init(struct AVXTEA *ctx, const uint8_t key[16]);
 
 /**
- * Encrypt or decrypt a buffer using a previously initialized context.
+ * Encrypt or decrypt a buffer using a previously initialized context,
+ * in big endian format.
  *
  * @param ctx an AVXTEA context
  * @param dst destination array, can be equal to src