input/csv: update comments/helptext for analog input data

Expand the developer comment that's inline in the source file. There is
enough room to explain things. Try to come up with a "one-line" help text
that is precise yet compact, and does inform the user of available format
choices including modifiers. Chances are that longer descriptions start
reducing the usefulness of the help or the visibility of options when
users are in a hurry. Those who care can access the manual.

Mark more options as obsolete, and mention more default values in the
builtin help text. Also tweak a comment on getting channel names from
header lines.