Add comments to StructureIsReused enum

Author: gluxon

src/compiler/types.ts

@@ -3822,10 +3822,11 @@ namespace ts {
     }
 
     /* @internal */
+    /** "Structure" refers to the SourceFile graph of a Program linked by module resolutions. */
     export const enum StructureIsReused {
-        Not         = 0,
-        SafeModules = 1 << 0,
-        Completely  = 1 << 1,
+        Not         = 0,      // The entire Program must be (re)created.
+        SafeModules = 1 << 0, // SourceFile objects need to be rediscovered, but module resolutions can be reused.
+        Completely  = 1 << 1, // SourceFile objects and module resolutions can be reused from an old Program.
     }
 
     export type CustomTransformerFactory = (context: TransformationContext) => CustomTransformer;