design: simplify "Coexist with flex driver" section
design: simplify "Coexist with flex driver" section

The "Coexist with flex driver" section was overly technical about
details in the implementation of CSI vs. flex. Remove the detail
and replace it with more goal oriented paragraphs.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

rook/rook,rook/nfs,rook/nfs,rook/rook,rook/rook,rook/nfs,rook/rook,rook/rook