doc: Clarify historical disclaimers in memory-barriers.txt

This commit makes it clear that the reason that these sections are
historical is that smp_read_barrier_depends() is no more.  It also
removes the point about comparison operations, given that there are
other optimizations that can break address dependencies.

Suggested-by: Jonas Oberhauser <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Alan Stern <[email protected]>
Cc: Andrea Parri <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: David Howells <[email protected]>
Cc: Jade Alglave <[email protected]>
Cc: Luc Maranget <[email protected]>
Cc: Akira Yokosawa <[email protected]>
Cc: Daniel Lustig <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: <[email protected]>
Cc: <[email protected]>
Signed-off-by: Neeraj Upadhyay (AMD) <[email protected]>

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