libusb20: Add dev_open + be_device_foreach manuals

Manual pages for libusb20_open(3) and libusb20_be_device_foreach(3).

PR: 291675
Relnotes: yes
MFC after: 3 days
Reviewed by: adrian (usb), ziaee (manpages)
Differential Revision: https://reviews.freebsd.org/D54231

(cherry picked from commit caf23c8e0b7981dd022c52feaf6edfef232f7e27)