Improve comments

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/include/mbedtls/cipher.h b/include/mbedtls/cipher.h
index a742b1a..006b0b1 100644
--- a/include/mbedtls/cipher.h
+++ b/include/mbedtls/cipher.h
@@ -282,13 +282,13 @@
     /** The block size, in bytes. */
     unsigned int MBEDTLS_PRIVATE(block_size) : 5;
 
-    /** IV or nonce size, in bytes (right shifted by 2).
+    /** IV or nonce size, in bytes (right shifted by MBEDTLS_IV_SIZE_SHIFT).
      * For ciphers that accept variable IV sizes,
      * this is the recommended size.
      */
     unsigned int MBEDTLS_PRIVATE(iv_size) : 3;
 
-    /** The cipher key length, in bits (right shifted by 6).
+    /** The cipher key length, in bits (right shifted by MBEDTLS_KEY_BITLEN_SHIFT).
      * This is the default length for variable sized ciphers.
      * Includes parity bits for ciphers like DES.
      */