Add documentation (doc lines) to variant reporting components

In order for the variant reporting lister to be really helpful, we need to add doc lines to modules and their parameters, as well as vcf_id and vcf_description to filters. The wording of the documentation is up for debate. Suggestions welcome!

This PR also fixes one minor bug and removes an unnecessary parameter.
It's weird to me that "snp" gets no parenthetical explanation here whereas "ins" and "del" do.  (Also, are these SNPs and not SNVs? (Do we still care about that distinction?))