Display component methods on the website and tweak the documentation

Summary:The website now displays public methods on components. This was implemented mostly in react-docgen via #66. This adds a <Method> component that is used by the component and API doc pages to display documentation for a method.

It also adds some missing documentation and tweak some existing one to integrate with this feature. I also prefixed some component methods with an '_' so they don't show up in the doc.

**Test plan (required)**

Tested every component page locally to make sure the methods doc was displayed properly.
Tested an API page to make sure it still worked properly.
Closes https://github.com/facebook/react-native/pull/6890

Differential Revision: D3159911

Pulled By: vjeux

fb-gh-sync-id: 1e6a4640cda6794496d9844c1af6a1451c017dcc
fbshipit-source-id: 1e6a4640cda6794496d9844c1af6a1451c017dcc

Libraries/Components/TextInput/TextInput.js

@@ -319,6 +319,9 @@ var TextInput = React.createClass({
         AndroidTextInput.viewConfig :
         {})) : Object),
 
+  /**
+   * Returns if the input is currently focused.
+   */
   isFocused: function(): boolean {
     return TextInputState.currentlyFocusedField() ===
       ReactNative.findNodeHandle(this.refs.input);
@@ -368,6 +371,9 @@ var TextInput = React.createClass({
     isInAParentText: React.PropTypes.bool
   },
 
+  /**
+   * Removes all text from the input.
+   */
   clear: function() {
     this.setNativeProps({text: ''});
   },