[texdoc] --help tweaks

[Karl Berry]

    I'd like to add some indication that you can sometimes use [NAME]-[suffix] 

Good.

    IIRC, the indentation and lines break are here to make help2man
    happy. I'll see what I can do on this side (suggestions welcome).

GNU coreutils (cp etc.) use help2man, and they indent by two spaces and
have the kind of trailer I wrote about.  I don't know if Jim (Meyering,
coreutils maintainer) does something special to make it happen.