manual: Clarify that Drop types cannot be Copy

Author: brson

doc/rust.md

@@ -2731,6 +2731,7 @@ The kinds are:
   : This kind includes all types that can be copied. All types with
     sendable kind are copyable, as are managed boxes, managed closures,
     trait types, and structural types built out of these.
+    Types with destructors (types that implement `Drop`) can not implement `Copy`.
 _Default_
   : Types with destructors, closure environments,
     and various other _non-first-class_ types,