Provide explicit docs for variable-specific config options

[jameshiebert]

We see a lot of questions where people get tripped up downscaling variables for which there are not default config options (i.e. variables other than 'tasmin', 'tasmax', 'pr'). It would be worthwhile to provide a paragraph and an example in the manual that specifically illustrates what you need to do in your pre-config to downscale non-default variables.