Document "providers" functionality

I have 2 other suggestions. In my opinion the module should implement its own hook, with the code for the 2 standard providers in there. Also, I think there should be a third standard provider, the default value. In the standard behavior it's possible to set a default value. This, however, isn't possible when enabling prepopulate. This causes an issue for usecases that do not require the URL parameter being present, where the selected default won't be respected.

@a.ross your suggestions sound good. Do I sense a patch coming? :)

Hehe you sensed right.
I already have done most of the work, but there is still an issue with the default value selection part of the field instance form disappearing when "prepopulate" is enabled. That results in a bad UX when a user needs to change the default, because then users will have to disable prepopulate, save, change the default, and finally re-enable prepopulate. I'm not sure what the best way is to fix this.

Found the code that basically removes the default widget, and that led me to a solution. Patch will follow soon.

I've made a separate issue out of this, because I haven't done a patch for end user documentation. #2249551: Improve providers functionality and add an extra provider