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[Switch Access] Add documentation

TBR=dmazzoni@chromium.org

AX-Relnotes: n/a.
Bug: None.
Change-Id: I242cdd57d3f762684c613a826e6787a26f18e35c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446657Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#813375}