vim-patch:8.2.0404: writefile() error does not give a hint

Problem:    Writefile() error does not give a hint.
Solution:   Add remark about first argument.
18a2b87ca2

src/nvim/eval/funcs.c

@@ -11857,7 +11857,8 @@ static void f_writefile(typval_T *argvars, typval_T *rettv, FunPtr fptr)
       }
     });
   } else if (argvars[0].v_type != VAR_BLOB) {
-    EMSG2(_(e_invarg2), "writefile()");
+    EMSG2(_(e_invarg2),
+          _("writefile() first argument must be a List or a Blob"));
     return;
   }