Update Error Message to Explain Namespace Restrictions for := and `…

…let` in `data.table` (#6302)

* udated the error message

* suggested wording

---------

Co-authored-by: nitish jha <nitishjha@nitishs-MacBook-Air.local>
Co-authored-by: Michael Chirico <chiricom@google.com>

R/data.table.R

@@ -2770,7 +2770,7 @@ address = function(x) .Call(Caddress, eval(substitute(x), parent.frame()))
 
 ":=" = function(...) {
   # this error is detected when eval'ing isub and replaced with a more helpful one when using := in i due to forgetting a comma, #4227
-  stopf('Check that is.data.table(DT) == TRUE. Otherwise, :=, `:=`(...) and let(...) are defined for use in j, once only and in particular ways. See help(":=").', class="dt_invalid_let_error")
+  stopf('Check that is.data.table(DT) == TRUE. Otherwise, :=, `:=`(...) and let(...) are defined for use in j, once only and in particular ways. Note that namespace-qualification like data.table::`:=`(...) is not supported. See help(":=").', class="dt_invalid_let_error")
 }
 
 # TODO(#6197): Export these.