Document that MANAGE_DOCUMENTS is optional

MR #151 implemented a fallback if MANAGE_DOCUMENTS is not granted,
which falls back to show a file chooser to the user to manually
select the storage root.

Document this behavior in the manifest as well, as the permission
causes some certification tests to fail (#453).

Change-Id: I3271b7969aacc014349766a022f13a00bea9ff85
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>
diff --git a/app/src/main/AndroidManifest.xml b/app/src/main/AndroidManifest.xml
index eadc02d..273be9e 100644
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -18,7 +18,8 @@
 
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
 
-    <!-- This is needed to retrieve the available storage roots -->
+    <!-- (Optional) This is needed to retrieve the available storage roots.
+         The user needs to manually select a storage root, if not granted. -->
     <uses-permission
         android:name="android.permission.MANAGE_DOCUMENTS"
         tools:ignore="ProtectedPermissions" />