documentation: add zram recompression documentation

documentation: add zram recompression documentation

Document user-space visible device attributes that are enabled by
ZRAM_MULTI_COMP.

Link: https://lkml.kernel.org/r/20221109115047.2921851-12-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Alexey Romanov <avromanov@sberdevices.ru>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>