Upvoted. I would also like to see more powerful search of just the helpx documents. But it has been improved. For example, go to the page you list, click in the search box at the top (which does say "Search Adobe Support"), and enter some search words. (I just used "mask.") The results page is filtered to Premiere Pro, and there are tabs over the right side. For me, these defaulted to "Support" and all links were to helpx/premiere pro. I have never gotten results that relevant.
More complicated searches may not get any relevant results.
edit58989569 was not looking for a way to search "Help", he was looking for a way to search "the Premiere Pro User Guide online."
You could interpret the proposed workarounds as adequate, or you could imagine than an easy, direct search limted to (and tuned to) the guide would be better and desirable.