[PATCH v3] locking/memory-barriers.txt: Improve documentation for writel() example

[Parav Pandit <parav@nvidia.com>]

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

Hence update the example to be more accurate that matches the current
implementation.

commit 5846581e3563 ("locking/memory-barriers.txt: Fix broken DMA vs. MMIO ordering example")

Signed-off-by: Parav Pandit <parav@nvidia.com>

---
changelog:
v2->v3:
- removed redundant description for writeX()
- updated text for alignment and smaller change lines
- updated commit log with blank line before signed-off-by line
v1->v2:
- Further improved description of writel() example
- changed commit subject from 'usage' to 'example'
v0->v1:
- Corrected to mention I/O barrier instead of dma_wmb().
- removed numbered references in commit log
- corrected typo 'explcit' to 'explicit' in commit log
---
 Documentation/memory-barriers.txt | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 832b5d36e279..8952fd86c6e6 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1927,10 +1927,11 @@ There are some more advanced barrier functions:
      before we read the data from the descriptor, and the dma_wmb() allows
      us to guarantee the data is written to the descriptor before the device
      can see it now has ownership.  The dma_mb() implies both a dma_rmb() and
-     a dma_wmb().  Note that, when using writel(), a prior wmb() is not needed
+     a dma_wmb().  Note that, when using writel(), a prior barrier is not needed
      to guarantee that the cache coherent memory writes have completed before
      writing to the MMIO region.  The cheaper writel_relaxed() does not provide
-     this guarantee and must not be used here.
+     this guarantee and must not be used here. Hence, writeX() is always
+     preferred.
 
      See the subsection "Kernel I/O barrier effects" for more information on
      relaxed I/O accessors and the Documentation/core-api/dma-api.rst file for
-- 
2.26.2