Add value list support to mp_drop. Refs #53.

doc/mp11/algorithm.adoc

@@ -320,6 +320,8 @@ As `mp_product`, but takes a quoted metafunction.
 
 `mp_drop_c<L, N>` removes the first `N` elements of `L` and returns the result.
 
+Supports a value list as `L` under {cpp}17.
+
 .mp_drop_c
 [cols="<.^4m,6*^.^1m",width=85%]
 |===