Documentation: android: binderfs: add 'stats' mount option

Add documentation of the binderfs 'stats' mount option.

Description taken from the commit message.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/baa0aa81-007d-af46-16a5-91fead0bd1b9@infradead.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/binderfs.rst

@@ -33,6 +33,12 @@ max
   a per-instance limit. If ``max=<count>`` is set then only ``<count>`` number
   of binder devices can be allocated in this binderfs instance.
 
+stats
+  Using ``stats=global`` enables global binder statistics.
+  ``stats=global`` is only available for a binderfs instance mounted in the
+  initial user namespace. An attempt to use the option to mount a binderfs
+  instance in another user namespace will return a permission error.
+
 Allocating binder Devices
 -------------------------