document android:elevation API compatibility

app/src/main/res/layout/episodes_apply_action_fragment.xml

@@ -1,5 +1,6 @@
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
+    xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent">
 
@@ -16,7 +17,14 @@
         android:layout_alignParentEnd="true"
         android:layout_alignParentRight="true"
         android:elevation="@dimen/sd_close_elevation"
-        > <!-- needs to match the speed dial's minimal elevation, or the speed dial can't be clicked at all -->
+        tools:ignore="UnusedAttribute">
+        <!-- android:elevation:
+             1. Needs to match the speed dial's minimal elevation,
+                or the speed dial can't be clicked at all
+             2. It is honored for API >= 21, but it does not hurt to be added here.
+                Use tools:ignore to suppress warnings.
+                Otherwise, we need a custom layout for API >= 21 that is more difficult to maintain.
+          -->
         <com.leinardi.android.speeddial.SpeedDialView
             android:id="@+id/fabSD"
             android:layout_width="match_parent"