Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Document the supported hardware, somewhat.

Reviewed by: hselasky
MFC after: 3 days

- Correct USB 3.0 wire-speed to 5.0Gbps

MFC after: 3 days
Approved by: thompsa (mentor)

- Add missing xhci(4) manual page.
- Minor update in some USB manual pages.

MFC after: 3 days
Approved by: thompsa (mentor)