add comments

packages/babel-generator/src/buffer.js

@@ -46,6 +46,8 @@ export default class Buffer {
     };
 
     if (map) {
+      // The `.map` property is lazy to allow callers to use the raw mappings
+      // without any overhead
       Object.defineProperty(result, "map", {
         configurable: true,
         enumerable: true,

packages/babel-generator/src/source-map.js

@@ -70,6 +70,8 @@ export default class SourceMap {
     this._lastSourceLine = line;
     this._lastSourceColumn = column;
 
+    // We are deliberately not using the `source-map` library here to allow
+    // callers to use these mappings without any overhead
     this._rawMappings.push({
       // undefined to allow for more compact json serialization
       name: identifierName || undefined,