Address review comments

Author: bracevac

docs/_docs/reference/experimental/capture-checking/classes.md

@@ -77,9 +77,11 @@ we know that the type of `this` must be pure, since `this` is the right hand sid
 
 ### Traits and Open Classes
 
-The self-type inference behaves differently depending on whether all subclasses of a class are known. For a regular (non-open, non-abstract) class, all subclasses are known at compile time, so the capture checker can precisely infer the self-type. However, for traits, abstract classes, and [`open`](../../other-new-features/open-classes.md) classes, arbitrary subclasses may exist, so the capture checker conservatively assumes that `this` may capture arbitrary capabilities
+The self-type inference behaves differently depending on whether all subclasses of a class are known. For a regular (non-open, non-abstract) class, all subclasses are known at compile time,¹ so the capture checker can precisely infer the self-type. However, for traits, abstract classes, and [`open`](../../other-new-features/open-classes.md) classes, arbitrary subclasses may exist, so the capture checker conservatively assumes that `this` may capture arbitrary capabilities
 (i.e., it infers the universal capture set `cap`).
 
+¹We ignore here the possibility that non-open classes have subclasses in other compilation units (e.g. for testing) and assume that these subclasses do not change the inferred self type.
+
 For example (assuming all definitions are in the same file):
 ```scala
 class A: