docs/source/android: add note about nested classes

Right after the first DPI example, write a short note demonstrating
the use of autoclass() with nested classes using the `$` character.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>

docs/source/android.rst

@@ -19,6 +19,8 @@ The `DisplayMetrics
     metrics = DisplayMetrics()
     print 'DPI', metrics.getDeviceDensity()
 
+Note: To access nested classes, use `$` like:
+`autoclass('android.provider.MediaStore$Images$Media')`.
 
 Recording an audio file
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