PCI: better document driver and device requirements

Feedback:
10) 4.1.3.1.2 Queue Vector Configuration
Some of the information from section 8.4 needs to be moved to
here, for example that the device may have an MSI-X table size
other than 2048.
Otherwise, this reads as though the MSI-X table must always have
2048 entries.

11) Please explicitly describe the device behavior when writing
a vector value beyond the MSI-X table size.

Address these comments.

Cc: Arun Subbarao <asubbarao@lnxw.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>