doc: update README.devices (USB detection, enable serial comm)

doc: update README.devices (USB detection, enable serial comm)

Slightly rephrase README.devices since there is no strict distinction
into "fully automatic" and "always manual". Some drivers _accept_ conn=
specs when provided, _and_ support automatic detection of enumerable
devices, _and_ might implement defaults but also support overrides.

Nit, numerically sort the list of Uni-T cables (which is about to grow
in the future) for improved readability.

Add two more devices which require users to manually enable the serial
communication (Brymen BM257s, EEVBlog 121GW).