- Act on existing TODO markers

Remove the restriction in classes.md: "The nullable type annotation, ?, can only be used on a type parameter that has the value type constraint, the reference type constraint without the nullable_type_annotation, or a class type constraint without the nullable_type_annotation."
Replace it with the C# 9 rule from the feature spec: unless a type parameter is explicitly constrained to value types, ? annotations can only be applied within a #nullable enable context.
Uncomment and integrate classes.md: "For a type parameter T when the type argument is a nullable reference type C?, instances of T? are interpreted as C?, not C??." (currently inside a <!-- Add in C# 9 --> comment).

— Override/explicit implementation constraint list

In classes.md, the text says: "Such declarations may only have type_parameter_constraints_clauses containing the primary_constraints class and struct…"
Add default to this list (e.g., "…the primary_constraints class, struct, and default…") and reference the new §15.6.5 and §19.6.2 meaning.

— Override method T? interpretation rules

In classes.md, the current rule only covers class and struct constraints and describes how T? is interpreted in overriding signatures. Update to add the default case:
If a default constraint is added for type parameter T, then T? represents the annotated type — a nullable reference type when T is a reference type, or just T when T is a value type (matching the feature spec behavior; no U??).
Consider adding an example analogous to the feature spec's A2/B2 example showing where T : default on an override of an unconstrained method.

— Explicit interface implementation T? interpretation rules

In interfaces.md, same situation as §15.6.5: update to add the default constraint case and its T? interpretation.
The text ("Without the type parameter constraint where T : class, the base method with the reference-typed type parameter cannot be overridden.") should be updated or supplemented with an example using where T : default for an unconstrained interface method.