Add explanations to discussion of private[this] in docs

docs/docs/reference/dropped-features/this-qualifier.md

@@ -12,6 +12,26 @@ Previously, these modifiers were needed for
 
  - avoiding the generation of getters and setters
  - excluding code under a `private[this]` from variance checks. (Scala 2 also excludes `protected[this]` but this was found to be unsound and was therefore removed).
+ - avoiding the generation of fields, if a `private[this] val` is not accessed
+ by a class method.
 
 The compiler now infers for `private` members the fact that they are only accessed via `this`. Such members are treated as if they had been declared `private[this]`. `protected[this]` is dropped without a replacement.
 
+This change can in some cases change the semantics of a Scala program, since a
+`private` val is no longer guaranteed to generate a field. The field
+is omitted if
+
+ - the `val` is only accessed via `this`, and
+ - the `val` is not accessed from a method in the current class.
+
+This can cause problems if a program tries to access the missing private field via reflection. The recommended fix is to declare the field instead to be qualified private with the enclosing class as qualifier. Example:
+```scala
+  class C(x: Int):
+    private[C] val field = x + 1
+      // [C] needed if `field` is to be accessed through reflection
+    val retained = field * field
+```
+
+
+
+