I feel you man, but the documentation that you linked, in the comment that you referenced with your capitalized explative doesn’t actually detail how to do it at all, and if you look at the forum, recent posts said and I quote GLOBAL STAGE/MULTI-TIMBRAL MODE IS DEAD!
I couldn’t find the menu and so I made a post called ‘Multi-Timbral mode after removal’, since I’m using the testing version after all.
After specifying the title, I described that I want my channels to be able to respond to any midi channel at any time, which is what multi timbral mode does, a feature I knew and loved, lol.
Turns out, after multi-timbral mode was removed in favor of four options, but it appears that the four options was after that removed, and the words multi timbral mode was placed back in, but under a long-press submenu, I have read all of the development threads, but the one guy complaining about it recently had a suggestion to bold-press the menu item, the thread is here, and it’s the only way on the internet right now to find out how to get there, even this thread didn’t help for 22 days, so I’m as surprised as you are.
Ironically its strongly encouraged in the thread with the answer, to ask lots of questions because v5 is new. And his problem was resolved after 5 monts, we’ve gone from 5 months to 22 days that’s big progress.
This is not yet detailed in the manual very well, this is why the thread exists, and now that the information is documented in the community to the degree that it was documented, sparsely, in another thread I found last night, I’m sure it will be included in the very rtfm you referenced 
I’d add it myself but there doesn’t seem to be a way to make an account on the site…
Next time you get offended when someone asks a question, write a song about it! I will listen to and thumbs up anything you make