Fix documentation for --lines

[hoelzro]

The documentation for --lines currently says:

  --lines=NUM                   Only print line(s) NUM of each file.  No
                                pattern is matched.

--lines=NUM implies only a single number may be specified, but line(s) NUM implies one may supply several. Not only is this inconsistent, but it doesn't explain how one would specify multiple lines if they desired.

For reference, you can specify a comma-separated list of expressions, each of which is a single integer line number, or a dash-separated start-end range, which matches all lines between start and end, inclusive.

[hoelzro]

Not really needed if we do #167

[petdance]

The --lines option no longer exists.