Add module level documentation for the `query_source` module
Add module level documentation for the `query_source` module

I don't want to hide the stuff in there, but we should indicate that
those types are mostly internal.

tohou/diesel,tohou/diesel,sgrif/diesel,diesel-rs/diesel,cascalheira/diesel,sgrif/diesel,mcasper/diesel,SoftwareWarlock/diesel,diesel-rs/diesel,mcasper/diesel,cascalheira/diesel,SoftwareWarlock/diesel