docs/memory-barriers.txt: Fix confusing name of 'data dependency barrier'

The term "data dependency barrier", which has been in
memory-barriers.txt ever since it was first authored by David Howells,
has become confusing due to the fact that in LKMM's explanations.txt
and elsewhere, "data dependency" is used mostly for load-to-store data
dependency.

To prevent further confusions, do the changes listed below:

  - substitute "data dependency barrier" with "address-dependency
    barrier";
  - add note on the removal of kernel APIs for explicit address-
    dependency barriers in kernel release v5.9;
  - note that address-dependency barriers are not necessary for
    load-to-store situations;
  - use READ_ONCE_OLD() for pre-4.15 READ_ONCE() (no implicit address-
    dependency barrier);
  - fix count of kernel memory barrier APIs;
  - and a few more context adjustments.

Note: Cleanups of long lines are deferred to a followup patch.
Reported-by: "Michael S. Tsirkin" <mst@redhat.com>
Link: https://lore.kernel.org/r/20211011064233-mutt-send-email-mst@kernel.org/Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>