docs: say what the availability comparison covers

The comparison now lists pages across the documentation, grouped as in
the navigation. The page says what Yes, No and Varies mean, that pages
spanning several features are left out, and that the SQL row stands for
the SQL reference.

docs/web/basics/offerings.mdx

@@ -28,7 +28,13 @@ Every page stays in the same navigation, whichever offering you use.
 
 ## Feature availability {#availability}
 
-These are the pages that declare where they apply.
+These are the pages that state where they apply, grouped as in the navigation.
+**Yes** means an offering has what the page describes and **No** that it does
+not; **Varies** means it depends on the version or configuration of a
+deployment, and the summary says on what. The list is not every feature:
+pages that span several, such as overviews and tutorials, are left out, and the
+[SQL](/basics/sql) row stands for the SQL reference, whose pages each say where
+they run.
 
 {/* availability-comparison */}