docs: Add documentation for rude and trace RCU flavors

This commit belatedly adds documentation of Tasks Rude RCU and Tasks
Trace RCU to RCU's requirements document.

Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>