Custom timeline settings - how to use that?
That particular menu is only available on the cut page, AFAIK.  The BMD website is referring to the fact that as of R16, projects can contain multiple timelines with different resolutions.  In previous versions of Resolve, timeline resolution was set at the project level; you could have multiple timelines in a project, but they all had to be the same resolution.

But if all you need is that little menu, here are the steps to find it: 

Switch to the "Cut" page.  It's icon looks like this:



Then click on the Icon in the upper-right side of the screen that looks like this:



This little window is really just a shortcut, which sets a few things in the project settings to commonly used values.  To put it another way, changing the "Timeline resolution" dropdown menu in Project Settings->Master Settings (gear menu) to 1920x1080, 3840x2160, etc, does exactly the same thing.

There are a few other places in Resolve's interface where you'll find shortcuts to the project settings, but all the magic really happens under the gear icon.  For example, when you create a new timeline via File->New Timeline, then click the "Custom Settings" button, and switch to the "Format" tab, you'll get a dialoge that looks like this: 

Again, these settings are just shortcuts to the main project settings, and you can change everything besides timeline framerate after the fact (which actually is possible, but that's another topic.  Best to just get it right the first time).

And just for good measure, here's a screenshot of the master "Project Settings", which you'll find under the gear menu on the bottom right of the UI: