doc: Remove kernel-parameters.txt entry for rcutorture.read_exit

There is only ever the one read-exit task, and there is no module
parameter named rcutorture.read_exit, so remove the bogus documentation.
Instead, use rcutorture.read_exit_burst to enable/disable read-exit
race testing.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>