Specify RAID1 as a mirror
Specify RAID1 as a mirror

The previous text raised a bunch of questions. Let's just say RAID1 is
mirrored for now.

Also shorten RAID6 description a little.

fixes #36

Signed-off-by: Andy Grover <b7d524d2f5cc5aebadb6b92b08d3ab26911cde33@redhat.com>

stratis-storage/stratisd,stratis-storage/stratisd,mulkieran/stratisd,trgill/stratisd,stratis-storage/stratisd,trgill/stratisd,mulkieran/stratisd