docs: binderfs: add section about feature files

Document how binder feature files can be used to determine whether a
feature is supported by the binder driver. "oneway_spam_detection" is
used as an example as it is the first available feature file.

Acked-by: Christian Brauner <[e-mail]>
Signed-off-by: Carlos Llamas <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/admin-guide/binderfs.rst

···
 ``binder-control`` device cannot be deleted since this would make the binderfs
 instance unusable.  The ``binder-control`` device will be deleted when the
 binderfs instance is unmounted and all references to it have been dropped.
+
+Binder features
+---------------
+
+Assuming an instance of binderfs has been mounted at ``/dev/binderfs``, the
+features supported by the binder driver can be located under
+``/dev/binderfs/features/``. The presence of individual files can be tested
+to determine whether a particular feature is supported by the driver.
+
+Example::
+
+        cat /dev/binderfs/features/oneway_spam_detection
+        1