[ReactNative] CameraRoll docs

Summary:
In preparation for open sourcing React Native for Android, document which parts of the `CameraRoll` API are platform-specific.

Renders like this: http://imgur.com/rbpXHKf

We should improve docs generation for `@param`s.

Libraries/CameraRoll/CameraRoll.js

@@ -109,17 +109,28 @@ var getPhotosReturnChecker = createStrictShapeTypeChecker({
   }).isRequired,
 });
 
+/**
+ * `CameraRoll` provides access to the local camera roll / gallery.
+ */
 class CameraRoll {
 
   static GroupTypesOptions: Array<string>;
   static AssetTypeOptions: Array<string>;
   /**
-   * Saves the image with tag `tag` to the camera roll.
+   * Saves the image to the camera roll / gallery.
    *
-   * @param {string} tag - Can be any of the three kinds of tags we accept:
-   *                       1. URL
-   *                       2. assets-library tag
-   *                       3. tag returned from storing an image in memory
+   * @param {string} tag On Android, this is a local URI, such
+   * as `"file:///sdcard/img.png"`.
+   *
+   * On iOS, the tag can be one of the following:
+   *
+   *   - local URI
+   *   - assets-library tag
+   *   - a tag not maching any of the above, which means the image data will
+   * be stored in memory (and consume memory as long as the process is alive)
+   *
+   * @param successCallback Invoked with the value of `tag` on success.
+   * @param errorCallback Invoked with error message on error.
    */
   static saveImageWithTag(tag, successCallback, errorCallback) {
     invariant(
@@ -140,10 +151,10 @@ class CameraRoll {
    *  Invokes `callback` with photo identifier objects from the local camera
    *  roll of the device matching shape defined by `getPhotosReturnChecker`.
    *
-   *  @param {object} params - See `getPhotosParamChecker`.
-   *  @param {function} callback - Invoked with arg of shape defined by
-   *    `getPhotosReturnChecker` on success.
-   *  @param {function} errorCallback - Invoked with error message on error.
+   *  @param {object} params See `getPhotosParamChecker`.
+   *  @param {function} callback Invoked with arg of shape defined by
+   *  `getPhotosReturnChecker` on success.
+   *  @param {function} errorCallback Invoked with error message on error.
    */
   static getPhotos(params, callback, errorCallback) {
     var metaCallback = callback;