add a sentence about env=foo

Author: MichaelChirico

vignettes/datatable-programming.Rmd

@@ -156,6 +156,8 @@ substitute2(
 
 We can see in the output that both the functions names, as well as the names of the variables passed to those functions, have been replaced. We used `substitute2` for convenience. In this simple case, base R's `substitute` could have been used as well, though it would've required usage of `lapply(env, as.name)`.
 
+Note that `outer = sqrt`, i.e., providing the _symbol_ instead of the function as a _character string_ is supported, and sometimes required (e.g. for namespace-qualified names like `pkg::foo`), but that there are some subtle differences between the two approaches that are beyond the scope of this vignette.
+
 Now, to use substitution inside `[.data.table`, we don't need to call the `substitute2` function. As it is now being used internally, all we have to do is to provide `env` argument, the same way as we've provided it to the `substitute2` function in the example above. Substitution can be applied to the `i`, `j` and `by` (or `keyby`) arguments of the `[.data.table` method. Note that setting the `verbose` argument to `TRUE` can be used to print expressions after substitution is applied. This is very useful for debugging.
 
 Let's use the `iris` data set as a demonstration. Just as an example, let's pretend we want to compute the `Sepal.Hypotenuse`, treating the sepal width and length as if they were legs of a right triangle.