update comment for cryptex unsupported error

The error is still quite generic as there are currently
two different reason this could be returned, but it is an indication the error is cryptex specific.

include/srtp.h

@@ -216,7 +216,7 @@ typedef enum {
     srtp_err_status_pkt_idx_adv = 27,   /**< packet index advanced, reset    */
                                         /**< needed                          */
     srtp_err_status_buffer_small = 28,  /**< out buffer is too small         */
-    srtp_err_status_cryptex_unsupported = 29, /**< out buffer is too small */
+    srtp_err_status_cryptex_unsupported = 29, /**< unsupported cryptex operation */
 } srtp_err_status_t;
 
 typedef struct srtp_ctx_t_ srtp_ctx_t;