Add descriptions for each zinc phases

internal/compiler-bridge/src/main/scala/xsbt/CallbackGlobal.scala

@@ -102,6 +102,8 @@ sealed class ZincCompiler(settings: Settings, dreporter: DelegatingReporter, out
     val analyzer = new Analyzer(global)
     def newPhase(prev: Phase) = analyzer.newPhase(prev)
     def name = phaseName
+
+    def description = "analyzes the generated class files and maps them to sources"
   }
 
   /** Phase that extracts dependency information */
@@ -116,6 +118,8 @@ sealed class ZincCompiler(settings: Settings, dreporter: DelegatingReporter, out
     val dependency = new Dependency(global)
     def newPhase(prev: Phase) = dependency.newPhase(prev)
     def name = phaseName
+
+    def description = "extracts dependency information"
   }
 
   /**
@@ -136,13 +140,18 @@ sealed class ZincCompiler(settings: Settings, dreporter: DelegatingReporter, out
     val api = new API(global)
     def newPhase(prev: Phase) = api.newPhase(prev)
     def name = phaseName
+
+    def description = "constructs a representation of the public API"
   }
 
   override lazy val phaseDescriptors = {
     phasesSet += sbtAnalyzer
+    phasesDescMap(sbtAnalyzer) = sbtAnalyzer.description
     if (callback.enabled()) {
       phasesSet += sbtDependency
+      phasesDescMap(sbtDependency) = sbtDependency.description
       phasesSet += apiExtractor
+      phasesDescMap(apiExtractor) = apiExtractor.description
     }
     this.computePhaseDescriptors
   }