clarify that macros can't generate macros (#2024)

Author: jakemac53

working/macros/feature-specification.md

@@ -260,6 +260,10 @@ Here, macros contribute new types to the program—classes, typedefs, enums,
 etc. This is the only phase where a macro can introduce a new visible name into
 the top level scope.
 
+**Note**: Macro classes _cannot_ be generated in this way, but they can rely on
+macro generated declarations for their implementation. This ensures that all
+macros can be discovered prior to actually running any macros.
+
 Very little introspective power is provided in this phase. Since other macros
 may also be declaring new types, we can't even assume that all top-level
 identifiers can be resolved. You can see the *names* of types that are
@@ -317,6 +321,7 @@ See some example macros [here][examples].
 constructors are invoked, and their limitations.
 -  All macros must implement at least one of the `Macro` interfaces.
 -  Macros cannot be abstract.
+-  Macro classes cannot be generated by other macros.
 
 *Note: The Macro API is still being designed, and lives [here][api].*