aes: improve av_aes_crypt() documentation

Originally committed as revision 23908 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/aes.h

@@ -35,7 +35,7 @@ struct AVAES;
 int av_aes_init(struct AVAES *a, const uint8_t *key, int key_bits, int decrypt);
 
 /**
- * Encrypt / decrypt.
+ * Encrypt or decrypt a buffer using a previously initialized context.
  * @param count number of 16 byte blocks
  * @param dst destination array, can be equal to src
  * @param src source array, can be equal to dst