zsmalloc: document freeable stats

commit 618a8a91 upstream.

When freeable class stat was added to classes file (back in 2016) we
forgot to update zsmalloc documentation.  Fix that.

Link: https://lkml.kernel.org/r/20230325024631.2817153-3-senozhatsky@chromium.org
Fixes: 1120ed54

 ("mm/zsmalloc: add `freeable' column to pool stat")
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>