pci.4: wordsmith and add missing words

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