Add Dropped Feature doc page

Author: OlivierBlanvillain

docs/docs/reference/dropped-features/nonlocal-returns.md

@@ -0,0 +1,14 @@
+---
+layout: doc-page
+title: Deprecated: Nonlocal Returns
+---
+
+Returning from nested anonymous functions has been deprecated. Nonlocal returns are implemented by throwing and catching `scala.runtime.NonLocalReturnException`-s. This is rarely what is intendant by the programmer. It can be problematic because of the hidden performance cost of throwing and catching exceptions. Furthermore, it is a leaky implementation: a catch-all exception handler can intercept a `NonLocalReturnException`.
+
+A drop-in library replacement is provided in `scala.util.control.NonLocalReturns`:
+
+```scala
+import scala.util.control.NonLocalReturns._
+
+returning { ... throwReturn(x) ... }
+```

docs/sidebar.yml

@@ -159,6 +159,8 @@ sidebar:
               url: docs/reference/dropped-features/auto-apply.html
             - title: Weak Conformance
               url: docs/reference/dropped-features/weak-conformance.html
+            - title: Nonlocal Returns
+              url: docs/reference/dropped-features/nonlocal-returns.html
     - title: Contributing
       subsection:
         - title: Getting Started