Update doc comments

Author: odersky

compiler/src/dotty/tools/dotc/transform/CacheAliasImplicits.scala

@@ -18,28 +18,20 @@ object CacheAliasImplicits {
 
   /** Flags that disable caching */
   val NoCacheFlags =
-    StableRealizable |  // It's a simple forwarder, leave it as one
-    Exported            // Export forwarders are never cached
+    StableRealizable  // It's a simple forwarder, leave it as one
+    | Exported        // Export forwarders are never cached
 }
 
 /** This phase ensures that the right hand side of parameterless alias implicits
- *  is cached. It applies to all alias implicits that have neither type parameters
- *  nor a given clause. Example: The alias
+ *  is cached if necessary. Dually, it optimizes lazy vak alias implicit to be uncached
+ *  if that does not change runtime behavior.
  *
- *      TC = rhs
+ *  A definition does not need to be cached if its right hand side has a stable type
+ *  and is of one of them forms
  *
- *  is expanded before this phase to:
- *
- *      implicit def a: TC = rhs
- *
- *  It is then expanded further as follows:
- *
- *  1. If `rhs` is a simple name `x` (possibly with a `this.` prefix) that
- *     refers to a value, leave it as is.
- *
- *  2. Otherwise, replace the definition with
- *
- *      lazy implicit val a: TC = rhs
+ *    this
+ *    this.y
+ *    y
  */
 class CacheAliasImplicits extends MiniPhase with IdentityDenotTransformer { thisPhase =>
   import tpd._