docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section

The "KERNEL I/O BARRIER EFFECTS" section of memory-barriers.txt is vague,
x86-centric, out-of-date, incomplete and demonstrably incorrect in places.
This is largely because I/O ordering is a horrible can of worms, but also
because the document has stagnated as our understanding has evolved.

Attempt to address some of that, by rewriting the section based on
recent(-ish) discussions with Arnd, BenH and others. Maybe one day we'll
find a way to formalise this stuff, but for now let's at least try to
make the English easier to understand.

Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Andrea Parri <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Daniel Lustig <[email protected]>
Cc: David Howells <[email protected]>
Cc: Alan Stern <[email protected]>
Cc: "Maciej W. Rozycki" <[email protected]>
Cc: Mikulas Patocka <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

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