Updated JavaDoc of getColumn(..) method (#11179)

* Updated JavaDoc of getColumn(..) method

Fixes https://github.com/vaadin/framework/issues/9920

server/src/main/java/com/vaadin/ui/Grid.java

@@ -3010,6 +3010,9 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
     /**
      * Gets a {@link Column} of this grid by its identifying string.
      *
+     * When you use the Grid constructor with bean class, the columns are 
+     * initialised with columnId being the property name. 
+     *
      * @see Column#setId(String)
      *
      * @param columnId