Add some explanatory comments

Author: amcasey

src/compiler/builder.ts

@@ -113,6 +113,8 @@ namespace ts {
         currentAffectedFilesSignatures: ESMap<Path, string> | undefined;
         /**
          * Newly computed visible to outside referencedSet
+         * We need to store the updates separately in case the in-progress build is cancelled
+         * and we need to roll back.
          */
         currentAffectedFilesExportedModulesMap: BuilderState.ComputingExportedModulesMap | undefined;
         /**

src/compiler/builderState.ts

@@ -43,6 +43,8 @@ namespace ts {
         /**
          * Contains the map of exported modules ReferencedSet=exported module files from the file if module emit is enabled
          * Otherwise undefined
+         *
+         * This is equivalent to referencedMap, but for the emitted .d.ts file.
          */
         readonly exportedModulesMap: BuilderState.TwoWayMap<Path, Path> | undefined;