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" <[email protected]>
Link: https://lore.kernel.org/r/[email protected]/
Signed-off-by: Akira Yokosawa <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Alan Stern <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Andrea Parri <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: David Howells <[email protected]>
Cc: Daniel Lustig <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

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