Flesh out docs on destination fields, and link to them

Working on #1240928: META: Refactoring of file destination/field handlers, I'm trying to figure out how to document the available fields and options, which appear on migration detail screens with migrate_ui. For many of the file field options, it just seems too much to fully document them through the descriptions returned by fields(). A simple solution - have links in those descriptions to the drupal.org documentation. First step, of course, will be fleshing out that documentation...