Document that WebView is iOS-only until we open source the Android implementation

Summary: The absence of docs can be confusing:
https://twitter.com/40_thieves/status/659500246632939520

public

Reviewed By: davidaurelio

Differential Revision: D2595809

fb-gh-sync-id: b741263782ad579bb40e94a383992dca3a6e8040

Libraries/Components/WebView/WebView.android.js

@@ -31,6 +31,10 @@ var WebViewState = keyMirror({
   ERROR: null,
 });
 
+/**
+ * Note that WebView is only supported on iOS for now,
+ * see https://facebook.github.io/react-native/docs/known-issues.html
+ */
 var WebView = React.createClass({
 
   propTypes: {

Libraries/Components/WebView/WebView.ios.js

@@ -75,6 +75,12 @@ var defaultRenderError = (errorDomain, errorCode, errorDesc) => (
   </View>
 );
 
+/**
+ * Renders a native WebView.
+ *
+ * Note that WebView is only supported on iOS for now,
+ * see https://facebook.github.io/react-native/docs/known-issues.html
+ */
 var WebView = React.createClass({
   statics: {
     JSNavigationScheme: JSNavigationScheme,