Rather that hide their purpose in some dark, damp corner of Documentation/,
add some documentation to the default implementations.
Signed-off-by: NValentin Schneider <valentin.schneider@arm.com>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200731192016.7484-2-valentin.schneider@arm.com