message: docs(ui-shell): update doc of side nav
  Adds a documentation on application-level style that should be added
  when `<bx-side-nav>` is used along with `<bx-header>`.
  Refs
diff: @@ -30,6 +30,17 @@ Side nav in UI Shell consists of the following components:
  Unlike `<bx-dropdown>`, etc., user gesture of side nav item won't update the selection by itself. The assumption is that following the link in `<side-nav-link>`/`<side-nav-menu-item>` triggers routing in application, and application chooses which `<side-nav-link>`/`<side-nav-menu-item>` to set `active` attribute to, with the new URL.
  +### Using with header nav
  +
  +If `<bx-side-nav>` is used along with header (`<bx-header>`), the side nav should be pushed down. To do that, add below style to your application:
  +
  +```css
  +bx-header ~ bx-side-nav {
  + margin-top: 3rem;
  + height: calc(100% - 3rem);
  +}
  +```
  +
  ### `<bx-side-nav>` attributes and properties
  Note: For `boolean` attributes, `true` means simply setting the attribute (e.g. `<bx-side-nav expanded>`) and `false` means not setting the attribute (e.g. `<bx-side-nav>` without `expanded` attribute).
  