spec: Clarify BLOCK_STATUS reply details

spec: Clarify BLOCK_STATUS reply details

Our docs were inconsistent on whether a NBD_REPLY_TYPE_BLOCK_STATUS
reply chunk can exceed the client's requested length, and silent on
whether the lengths must be consistent when multiple contexts were
negotiated.  Clarify this to match existing practice as implemented in
qemu-nbd and nbdkit.  Clean up some nearby grammatical errors while at
it.

Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>