Try disabling OneDrive by going to the settings and choosing "Quit OneDrive." Then relocate the Documents folder so it's in the right place (c:\users[yourid]\documents. Then re-enable OneDrive and choose a custom location for it during that process. This is how I fixed the problem. After that, I could relocate my Documents folder to a different drive and it was no longer under the OneDrive folder.
Edit: The key is to "Unlink this PC." Then you can relocate Documents, Pictures, etc. Then go ahead and relink OneDrive. In my case, I put OneDrive on a separate internal drive instead of the system drive, and it's now well-behaved. Quite why MS decided to make this so difficult is a mystery. One of those bogus "user studies," no doubt. (See any Panos Panay presentation for a lesson in how not to upgrade a user interface.)