rcu: docs: Requirements.rst: Abide by conventions of kernel documentation

rcu: docs: Requirements.rst: Abide by conventions of kernel documentation

Here is a list of conventions applied here:

- Don't mark up function names, to be taken care of by the automarkup
  extension.  Just say func().
- Instead of ".. code-block:: none", just say "::".
- Mark inline literals by a pair of ``xxxx``.  Don't use rust doc's
  dialect of `yyyy`.
- Instead of emphasizing headings by **strong emphasis**, use sub-level
  title adornments, in this case "^^^^^^^^^^" and make them proper
  sub-sections under "Hotplug CPU".

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>