chore(api) improve documentation.

lib/api/src/externals/function.rs

@@ -948,6 +948,8 @@ mod inner {
                 }
             }
 
+            // Implement `HostFunction` for a function that has the same arity than the tuple.
+            // This specific function has an environment.
             #[allow(unused_parens)]
             impl< $( $x, )* Rets, RetsAsResult, Env, Func >
                 HostFunction<( $( $x ),* ), Rets, WithEnv, Env>