MAGETWO-37920: Add documentation for Styles Library components

- CR changes

Author: Bogdan Plieshka

lib/web/css/docs/source/variables.less

@@ -6047,7 +6047,7 @@
 //        <th>@component__shadow-size__base</th>
 //        <td class="vars_value">5px</td>
 //        <td class="vars_value">constant</td>
-//        <td>Shadow size, cross all components. Use to count overlay zones sizes</td>
+//        <td>Shadow size across all components. Used to count overlay zones sizes</td>
 //      </tr>
 //      <tr>
 //        <th>@component__box-shadow__base</th>