sf: doc: Add documentation for the 'sf' command

This command is fairly complicated so documentation is useful.
Unfortunately I an not sure how the MTD side of things works and cannot
find information about that.

Signed-off-by: Simon Glass <sjg@chromium.org>

Acked-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>