doc/rcuref: Document real world examples in kernel

Document similar real world examples in the kernel corresponding to the
second and third code snippets. Also correct an issue in
release_referenced() in the code snippet example.

Cc: oleg@redhat.com
Cc: jannh@google.com
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Do a bit of wordsmithing. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>