Document when usesCleartextTraffic is ignored.

In N and above when using an Android Network Security Config the
cleartext configuration from that is used and
android:usesCleartextTraffic in the manifest is ignored.

Bug:27596429
Change-Id: I50ec765cead6cfd6bbaec585723f99d72e0fd945

core/java/android/content/pm/ApplicationInfo.java

@@ -366,6 +366,9 @@ public class ApplicationInfo extends PackageItemInfo implements Parcelable {
      *
      * <p>NOTE: {@code WebView} does not honor this flag.
      *
+     * <p>This flag is ignored on Android N and above if an Android Network Security Config is
+     * present.
+     *
      * <p>This flag comes from
      * {@link android.R.styleable#AndroidManifestApplication_usesCleartextTraffic
      * android:usesCleartextTraffic} of the &lt;application&gt; tag.

docs/html/guide/topics/manifest/application-element.jd

@@ -472,6 +472,8 @@ from {@link android.content.pm.ApplicationInfo#flags ApplicationInfo.flags} or
 {@link android.os.StrictMode.VmPolicy.Builder#detectCleartextNetwork() StrictMode.VmPolicy.Builder.detectCleartextNetwork()}.
 
 <p>This attribute was added in API level 23.</p>
+
+<p>This flag is ignored on Android N and above if an Android Network Security Config is present.</p>
 </dd>
 
 <dt><a name="vmSafeMode"></a>{@code android:vmSafeMode}</dt>