Add comment to phase

Author: odersky

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -106,7 +106,7 @@ class Compiler {
     List(new ElimErasedValueType,    // Expand erased value types to their underlying implmementation types
          new PureStats,              // Remove pure stats from blocks
          new VCElideAllocations,     // Peep-hole optimization to eliminate unnecessary value class allocations
-         new EtaReduce,
+         new EtaReduce,              // Reduce eta expansions of pure paths to the underlying function reference
          new ArrayApply,             // Optimize `scala.Array.apply([....])` and `scala.Array.apply(..., [....])` into `[...]`
          new sjs.AddLocalJSFakeNews, // Adds fake new invocations to local JS classes in calls to `createLocalJSClass`
          new ElimPolyFunction,       // Rewrite PolyFunction subclasses to FunctionN subclasses