docs: Clarify options that should not have data

docs: Clarify options that should not have data

The spec indirectly stated that NBD_OPT_LIST should be sent without
data (by virtue of the fact that NBD_REP_ERR_INVALID mentioned
non-zero data length as a reason for failure), but making zero-length
options explicit in the documentation of each option is nicer.
As an exception, make it clear that NBD_OPT_ABORT should succeed at
ending the connection rather than being ignored with an error due
to non-zero length, even though well-behaved clients won't send such
length.

This mirrors existing practice in many implementations, and is done
in a way that can be copied to NBD_OPT_STRUCTURED_REPLY and other
extension options that do not require data.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Wouter Verhelst <w@uter.be>