Add documentation on planned inline changes.

^KT-64570

docs/inlining/examples/outer-this.md

@@ -0,0 +1,24 @@
+### Inner classes 
+
+The basic problematic example looks like this:
+
+```kotlin
+class A(val x: Int) {
+    inner class B {
+        inline fun foo() = x
+    }
+}
+
+fun main() {
+    A(5).B().foo()
+}
+```
+
+The problem is caused by the fact that there is no field, which stores A object inside B doesn't exist after
+Fir2IR. This field is added by lowering (which is now exectued before inlining, but probably should be moved after),
+and is a private one. This leads to two problems
+1. This field doesn't exist in klib
+2. Even if it exists, it shouldn't have a public signature, so can't be referenced from other klib.
+
+There is a possibility that we can deprecate accessing this of outer class from public inline functions.
+This would make fixing things a bit easier here.