There are just a few issues I've found that I'm sure Wolfram will address, but knowing them might save others some of the churn that I went through thinking I wasn't doing it right.

1. Double/multi-usage lines don't quite format correctly when creating Symbol reference pages (see: https://community.wolfram.com/groups/-/m/t/2436487)
2. When building documentation, the Tech Notes section containing Tech Note (Tutorial) links formats correctly on Guide pages, but not on Symbol reference pages. Hopefully this will get fixed soon. Here's the result on my sample Guide and Symbol pages. ===========================================================
3. The hover button on paclet/package function symbols contains the double-down-chevron that will bring up the usage statement, but the info button (that should link to the Symbol reference page) is still grayed out and unusable.
4. However, if you request the symbol definition with the "?" prefix, the info button on that usage statement is active and does link to the Symbol reference page.

Other than these minor issue, the new DocumentationTools work really well and generate seamlessly integrated documentation pages for Paclets/Packages. I'm in the process of converting a big project that was originally created under Wolfram Workbench and most pages build well with the new DocumentationTools.