Document PCI_HP and PCI_IOV kernel options and various tunables in pci(4).

Describe PCI-related kernel options for HotPlug and SR-IOV support in the
pci(4) manual page. While here, add a section describing the various
tunables supported by the PCI bus driver as well.

Reviewed by: wblock
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D7754