pciconf.8: Reorganize slightly to handle additional modes

Move the description of the optional device argument earlier before
describing individual command modes.

Add a subsection for list mode and a second subsection for the other
modes that work with a single device.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D55773

(cherry picked from commit 98a0d2283701e08353ce670c8023803c58a4994c)