docs: ABI: Drop trailing whitespace

Remove all trailing whitespace from the ABI documentation.
Most of it was introduced during recent updates.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201110144033.3278499-1-geert+renesas@glider.be


Signed-off-by: Jonathan Corbet <corbet@lwn.net>