More help on editing field mappings

With Migrate 2.6 we'll allow overriding field mappings through the UI. The full list of available fields/options/subfields is intimidating, and at the moment we're simply not showing some of the more obscure ones, but I think that's the wrong approach. I'm going to restore them, but add in links to documentation for subfields/options where it's missing to help guide them. Also, I want to visually distinguish the subfields/options from top-level fields (perhaps simply by indenting them).