Android Studio 0.8.2如何附加javadoc支持库

我需要在我的项目中附加 javadoc for support library v4：20.我的libs文件夹是空的.它不包含支持库.我的项目正在运行,但支持库不是javadoc.我的build.gradle是：

apply plugin: 'android'

android {
    compileSdkVersion 20
    buildToolsVersion '20.0.0'

    defaultConfig {
        applicationId "com.myapp"
        minSdkVersion 14
        targetSdkVersion 20
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            runProguard false
            proguardFiles getDefaultProguardFile('proguard-android.txt'),'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs',include: ['*.jar'])
    // You must install or update the Support Repository through the SDK manager to use this dependency.
    compile 'com.android.support:support-v4:20.+'
    compile 'com.android.support:appcompat-v7:20.+'
}

以这种方式添加了库支持：