How to view function signature or go to external documentation source from Quick Documentation

While working in the console I'd like to get Quick Documentation for a built-in DB function and I can bring up the Quick Documentation view, but the view doesn't actually display the function signature and the up arrow is greyed out / disabled so I can't go to the external doc source.  For example, I'm connected to a PostgreSQL db and use date_trunc() in my query.  When I open quick documentation the window says only "Truncate to specified precision; see also Section 9.9.2" but doesn't display the function signature.  Luckily while typing it, the auto-complete will show the signature, but sometimes you'll be returning to the already typed function and auto-complete won't be available there so it'd be great to have that in the documentation view as well.

Related, is there any way to configure the external documentation source so that I can quickly jump to it from the Quick Doc view?