Several documentation changes.

First, we never mentioned any errors and never required any
error checks in documentation.

Second, mention that there is another way to configure and
manage zram devices -- zramctl, provided by util-linux.

Third, add a bit of clarification on why `mem_used_max' attr
is RW and correct some typos.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>