?dstrfw should be more upfront about the use of col_types names

[MichaelChirico]

By complete accident doing some debugging, I realized that (despite my having read the argument list in ?dstrfw several times), contrary to what I had been doing, dstrfw provides functionality to handle input column names directly (rather than fixing up ex-post).

I only saw this because I was reading the source code of the function -- it took very careful poring over every word of the rest of ?dstrfw to find the morsel at the end of the Value section which states this feature.

It's a great feature, so it should get higher prominence! In particular it should definitely get mention in the col_types argument explanation.

I'm happy to submit a PR to this end.