Revise block_evaluate method documentation
Revise block_evaluate method documentation

Make it more like a precise specification and less like a description of
what it usually does.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd