docs: update typography with headings a11y (#120)

Author: rlax

docs/components/Typography.mdx

@@ -19,7 +19,7 @@ All elements of Smooth UI have defaults font settings that you can override in t
 
 ## Headings
 
-Six size of headings are available.
+A top-level heading (h1) and five subsection headings (h2-h6) are available. Please use these in accordance with [current best practices](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Heading_Elements). They are not intended to control font size: use style properties for that when necessary.
 
 <Playground>
   <BlockList>