Add clarifying note to layout docs (#800)

Author: tcbegley

docs/components_page/components/layout.md

@@ -5,6 +5,8 @@ lead: Components for laying out your Dash app, including wrapping containers, an
 
 ## Examples
 
+**Note:** the rows and columns in the examples below have had additional styles applied to them with CSS to make sure that the effects of different arguments are clear. If you copy the snippets your rows and columns will not have these extra styles applied. Specifically the contents will not have a border and background colour, the rows will not have a background colour or bottom margin.
+
 Layout in Bootstrap is controlled using the grid system. The Bootstrap grid has twelve columns, and six responsive tiers (allowing you to specify different behaviours on different screen sizes, see below). The width of your columns can be specified in terms of how many of the twelve grid columns it should span, or you can allow the columns to expand or shrink to fit either their content or the available space in the row.
 
 There are three main layout components in _dash-bootstrap-components_: `Container`, `Row`, and `Col`.