docs: Expand the "BIOS bootloader" documentation for domainCaps

docs: Expand the "BIOS bootloader" documentation for domainCaps

Rewrite some parts for clarity, elaborate the meaning of some of the XML
attributes.  And where necessary, distinguish that we're dealing with
two different XML documents here:

  - the domainCapabilities XML, to detect the host "hypervisor"
    (QEMU/KVM) capabilities, and what libvirt knows about them.

  - the guest XML definition, i.e. what features a guest can use, based
    on the capabilities (of QEMU and libvirt and the host) reported in
    the domainCapabilities XML.

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>