List some gmp-6 functions to take advantage of.

misc/plan.html

@@ -86,7 +86,9 @@
   
   <h2> Miscellaneous </h2>
   <p class='should'>
-    Use more GMP-6 functions when available.
+    Use more functions from GMP-6 and later, when available:
+    mpn_sec_add_1, mpn_sec_tabselect, mpn_sec_invert, mpn_cnd_swap,
+    ...
   </p>
 
   <h2> Documentation </h2>