Fixing JavaDoc of Column#getWidth() and Column#setWidth() (#11452)

Fixes #11451

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

         }
 
         /**
-         * Returns the width (in pixels). By default a column is 100px wide.
+         * Returns the width (in pixels). By default a column width is 
+         * {@value com.vaadin.shared.ui.grid.GridConstants#DEFAULT_COLUMN_WIDTH_PX}
+         * (undefined).
          *
          * @return the width in pixels of the column
          * @throws IllegalStateException
          * This defines the minimum guaranteed pixel width of the column
          * <em>when it is set to expand</em>.
          *
+         * Note: Value -1 is not accepted, use {@link #setWidthUndefined()}
+         * instead.
+         *
          * @param pixels
          *            the minimum width for the column
          * @throws IllegalStateException