SE-0400: State limitations (#2389)

* SE-0400: `init` accessors cannot be declared outside primary type declaration

* SE-0400: `init` accessors can be ABI

Author: AnthonyLatsis

proposals/0400-init-accessors.md

@@ -491,13 +491,21 @@ init() {
 }
 ```
 
+### Restrictions
+
+A property with an `init` accessor can only be declared in the primary
+declaration of a type.
+
 ## Source compatibility
 
 `init` accessors are an additive capability with new syntax; there is no impact on existing source code.
 
 ## ABI compatibility
 
-`init` accessors are only called from within a module, so they are not part of the module's ABI. In cases where a type's initializer is `@inlinable`, the body of an `init` accessor must also be inlinable.
+`init` accessors are an ABI-additive change; they are at most `internal` but can
+be ABI-public.
+Calling an `init` accessor from an `inlinable` type initializer requires that
+the `init` accessor is ABI-public.
 
 ## Implications on adoption