tools/memory-model: docs/ordering: Fix trivial typos

Fix trivial typos including:

  - Repeated "a call to"
  - Inconsistent forms of referencing functions of rcu_dereference()
    and rcu_assign_pointer()
  - Past tense used in describing normal behavior

and other minor ones.

[ paulmck: Wordsmith plus recent LWN RCU API URL. ]

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Andrea Parri <parri.andrea@gmail.com>