subtype -> nested types.

Author: gottesmm

proposals/0000-compiler-settings.md

@@ -184,9 +184,9 @@ can be one of:
   synthesizes cases for depending on internal compiler data (e.x.: features for
   `UpcomingFeatureCompilerSetting`)
 
-We purposely keep all custom types defined for `CompilerSetting` as subtypes of
-`CompilerSetting` in order to avoid polluting the global namespace. Thus we
-would necessarily define in the stdlib all of the custom types as:
+We purposely keep all custom types defined for `CompilerSetting` as nested types
+within `CompilerSetting` in order to avoid polluting the global namespace. Thus
+we would necessarily define in the stdlib all of the custom types as:
 
 ```swift
 extension CompilerSetting {
@@ -238,7 +238,7 @@ where one must have a keyword preventing any ambiguity. Adding the enum
 added to the compiler for `Result` will also guarantee that any user defined
 `CompilerSetting` will take precedence over the `CompilerSetting` type. In such
 a situation, the user can spell `CompilerSetting` as `Swift.CompilerSetting` as
-needed. All of the subtypes of `CompilerSetting` cannot cause any source
+needed. All of the nested types within `CompilerSetting` cannot cause any source
 compatibility issues since they are namespaced within `CompilerSetting`.
 
 ## ABI compatibility