UPSTREAM: zram: doc fixes

(cherry-pick from commit 51d8a7b0a04effbb67fd9afcda4757b0c061fc11)

Simple doc updates to zram documentation.

Bug: 25951511

Change-Id: I08256de92d7209a345967e5a1573591fe692a3c9
Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>