Add missing words after PCI in the description of the PCIOCWRITE and
PCIOCATTACHED ioctls.
Use singular in PCIOCREAD, we only read one register at the time.
Reviewed by: bcr, bjk, rgrimes, cem
MFC after: 2 weeks
X-MFC-with: r349133
Differential Revision: https://reviews.freebsd.org/D20671