Add documentation for withsource parameter to union page (#217)

Author: benenharrington

apl/tabular-operators/union-operator.mdx

@@ -96,6 +96,7 @@ T1 | union [T2], [T3], ...
 ### Parameters
 
 - `T1, T2, T3, ...`: Tables or query results you want to combine into a single output.
+- `withsource`: An optional parameter that allows you to specify the name of a column in the output whose values indicate the source dataset of each row.
 
 ### Returns