Expand documentation scroll to offset

Summary:
I expanded the documentation of the scrollToOffset method a bit in FlatList

Thanks for submitting a PR! Please read these instructions carefully:

- [x] Explain the **motivation** for making this change.
- [x] Provide a **test plan** demonstrating that the code is solid.
- [x] Match the **code formatting** of the rest of the codebase.
- [x] Target the `master` branch, NOT a "stable" branch.

The documentation of the scrollToOffset method in FlatList was a bit confusing. Also, there was no documentation for this method on VirtualizedList. I added the doc block there and linked to it from the FlatList doc block.

Please check if the link on the FlatList scrollToOffset method links to the scrollToOffset method of the VirtualizedList docs page.

Sign the [CLA][2], if you haven't already.

Small pull requests are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.

Make sure all **tests pass** on both [Travis][3] and [Circle CI][4]. PRs that break tests are unlikely to be merged.

For more info, see the ["Pull Requests"][5] section of our "Contributing" guidelines.

[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: https://travis-ci.org/facebook/react-native
[4]: http://circleci.com/gh/facebook/react-native
[5]: https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests
Closes https://github.com/facebook/react-native/pull/14317

Differential Revision: D5192378

Pulled By: hramos

fbshipit-source-id: 6131a8e5adb2a2f7cb84344541f4f8b999a232d2

Libraries/Lists/FlatList.js

@@ -325,7 +325,9 @@ class FlatList<ItemT> extends React.PureComponent<DefaultProps, Props<ItemT>, vo
   }
 
   /**
-   * Scroll to a specific content pixel offset, like a normal `ScrollView`.
+   * Scroll to a specific content pixel offset in the list.
+   * 
+   * Check out [scrollToOffset](docs/virtualizedlist.html#scrolltooffset) of VirtualizedList
    */
   scrollToOffset(params: {animated?: ?boolean, offset: number}) {
     this._listRef.scrollToOffset(params);

Libraries/Lists/VirtualizedList.js

@@ -238,6 +238,16 @@ class VirtualizedList extends React.PureComponent<OptionalProps, Props, State> {
     }
   }
 
+  /**
+   * Scroll to a specific content pixel offset in the list.
+   * 
+   * Param `offset` expects the offset to scroll to.
+   * In case of `horizontal` is true, the offset is the x-value,
+   * in any other case the offset is the y-value.
+   * 
+   * Param `animated` (`true` by default) defines whether the list
+   * should do an animation while scrolling.
+   */
   scrollToOffset(params: {animated?: ?boolean, offset: number}) {
     const {animated, offset} = params;
     this._scrollRef.scrollTo(