Document validity of --assume-clean when all devices are zeroed.

Document validity of --assume-clean when all devices are zeroed.

>>
>> When I assemble an array I tend to have checked the devices before
>> hand; it would not be difficult to make the final pass a zeroing pass
>> if I knew I could vastly speed up post-assembly performance.  As I
>> stated, it's merely a lack of clarity in the documentation.
>
> If you would like to create a patch against the man page, I would be happy to
> accept it.
>
> NeilBrown

Signed-off-by: NeilBrown <neilb@suse.de>