Accessibility: Improve tips for crossing header styles

Created by: courier-new

I ran into a situation with the same root problem as https://github.com/sourcegraph/sourcegraph/pull/35776, where I needed to "upscale" a header so that an <h3> has the styles of an <h2>, but I didn't realize <H2 as={H3}> doesn't work that direction and thought I had hit a bug. In this PR, I tried to clarify in the docs that we currently need to use a different API for upscaling vs. downscaling and added a Storybook story to demonstrate crossing styles both directions. 🙂

Test plan

Just Storybook and docs changes.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.