Various documentation cleanups.

Options are shortest unique prefices, so say so.  Then fix the
documented `--check` to be its full `--checkscript`.

Describe in the _docs_ what's meant by a Perl port specification and
what the implications are.

Drop the patronising text around the action options; how embarrassing,
that I once wrote such garbage.  Instead, actually bother documenting
the options.

Fix the ENVIRONMENT section to reference all the vars now honoured.

Reference Mozilla::PublicSuffix in PREREQUISITES.

Various typos cleaned.