Update comments

toolchain/check/context.cpp

@@ -493,9 +493,6 @@ auto Context::LookupQualifiedName(SemIRLoc loc, SemIR::NameId name_id,
 
     // If this is our second lookup result, diagnose an ambiguity.
     if (result.inst_id.is_valid()) {
-      // TODO: This is currently not reachable because the only scope that can
-      // extend is a class scope, and it can only extend a single base class.
-      // Add test coverage once this is possible.
       CARBON_DIAGNOSTIC(
           NameAmbiguousDueToExtend, Error,
           "ambiguous use of name `{0}` found in multiple extended scopes",

toolchain/sem_ir/name_scope.h

@@ -87,8 +87,7 @@ struct NameScope : Printable<NameScope> {
   // needs to describe the `T*` argument.
   //
   // Small vector size is set to 1: we expect that there will rarely be more
-  // than a single extended scope. Currently the only kind of extended scope is
-  // a base class, and there can be only one of those per scope.
+  // than a single extended scope.
   // TODO: Revisit this once we have more kinds of extended scope and data.
   // TODO: Consider using something like `TinyPtrVector` for this.
   llvm::SmallVector<NameScopeId, 1> extended_scopes;