locking/memory-barriers.txt: Improve documentation for writel() example

The cited commit describes that when using writel(), explicit wmb()
is not needed. wmb() is an expensive barrier. writel() uses the needed
platform specific barrier instead of wmb().

writeX() section of "KERNEL I/O BARRIER EFFECTS" already describes
ordering of I/O accessors with MMIO writes.

Hence add the comment for pseudo code of writel() and remove confusing
text around writel() and wmb().

commit 5846581e ("locking/memory-barriers.txt: Fix broken DMA vs. MMIO ordering example")

Co-developed-by: Will Deacon <will@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>