Descriptive comment for TwinColSelect - no lazy load #11025 (#11350)

* Descriptive comment for TwinColSelect - no lazy load #11025

It documents the issue #11025

https://github.com/vaadin/framework/issues/11025#issuecomment-443506307

documentation/components/components-twincolselect.asciidoc

@@ -59,6 +59,11 @@ The [methodname]#setRows()# method sets the height of the component by the
 number of visible items in the selection boxes. Setting the height with
 [methodname]#setHeight()# to a defined value overrides the rows setting.
 
+[WARNING]
+The [classname]#TwinColSelect# does not provide lazy loading mechanism.
+Hence it will slow down significantly if used with large itemsets.
+The lazy loading feature could be implemented using two single column Grids instead.
+
 Common selection component features are described in
 <<dummy/../../../framework/components/components-selection#components.selection,"Selection
 Components">>.