Update FlatList doc

Summary:
1. Mention scrollToIndex support.
2. Fix a format error.
3. Use relative links like https://github.com/facebook/react-native/blob/59257d6976656f4eeaf389ae5527dfaff08e7354/Libraries/Components/Navigation/NavigatorIOS.ios.js#L136 for links in "next version" to work properly.

~~4. BREAKING: `columnWrapperStyle` should be `rowWrapperStyle`?~~
Closes #12714

Differential Revision: D4730476

Pulled By: sahrens

fbshipit-source-id: 643aab659064e6f5275ec89fd7f967dd9de866c4

Author: sunnylqm

Libraries/CustomComponents/Lists/FlatList.js

@@ -158,8 +158,9 @@ type DefaultProps = typeof defaultProps;
  *  - Separator support.
  *  - Pull to Refresh.
  *  - Scroll loading.
+ *  - ScrollToIndex support.
  *
- * If you need section support, use [`<SectionList>`](/react-native/docs/sectionlist.html).
+ * If you need section support, use [`<SectionList>`](docs/sectionlist.html).
  *
  * Minimal Example:
  *
@@ -168,7 +169,7 @@ type DefaultProps = typeof defaultProps;
  *       renderItem={({item}) => <Text>{item.key}</Text>}
  *     />
  *
- * This is a convenience wrapper around [`<VirtualizedList>`](/react-native/docs/virtualizedlist.html),
+ * This is a convenience wrapper around [`<VirtualizedList>`](docs/virtualizedlist.html),
  * and thus inherits the following caveats:
  *
  * - Internal state is not preserved when content scrolls out of the render window. Make sure all