doc: update sigrok-cli(1) for channel assignment to decoder inputs

doc: update sigrok-cli(1) for channel assignment to decoder inputs

Extend the sigrok-cli manpage section which discusses the -P option.
Mention the assignment of logic channels to decoder inputs by index
or by name.

Automatic assignment by position in the absence of user specs seems
to not have been recent implementations' default behaviour. Remove
that outdated or incorrect comment.