Update vignettes/datatable-joins.Rmd

Co-authored-by: Michael Chirico <[email protected]>

Author: venom1204

vignettes/datatable-joins.Rmd

@@ -577,8 +577,10 @@ When performing non-equi joins (`<`, `>`, `<=`, `>=`), column names are assigned
 - The right operand (`i` column) contributes values but does not retain its original name.
 - By default, `data.table` does not retain the `i` column used in the join condition unless explicitly requested.
 
-In non-equi joins, the left side of the operator (e.g., x_int in x_int >= i_int) must be a column from x, while the right side (e.g., i_int) must be a column from i. Non-equi joins do not support arbitrary expressions.
-For example, on = .(x_int >= i_int) is valid, but on = .(x_int >= i_int + 1) is not valid.
+In non-equi joins, the left side of the operator (e.g., `x_int` in `x_int >= i_int`) must be a column from `x`, while the right side (e.g., `i_int`) must be a column from `i`. Non-equi joins do not support arbitrary expressions.
+For example, `on = .(x_int >= i_int)` is valid, but `on = .(x_int >= i_int + 1)` is not valid.
+
+We plan to allow arbitrary expressions eventually, see [#1639](https://github.com/Rdatatable/data.table/issues/1639).
 
 ```{r}
 x <- data.table(x_int = 2:4, lower = letters[1:3])