Resolved Moving Documents in 'This PC Documents'

"This PC > Documents" is actually your user profile Documents, located at C:\YOUR USERNAME\Documents.
On the D drive create a folder called Documents.
In the left pane, right click on This PC > Documents, then left click on Properties.
In Properties windows click on Location tab.
Click the Move button and select the destination as the new Documents folder on the D drive.
Follow the prompts to move all files from C:\YOUR USERNAME\Documents to D:\Documents.

Doing it this way Windows will now know that your user account Documents are on the D drive.
You can do the same for any folder in C:\YOUR USERNAME\ such as C:\YOUR USERNAME\Desktop or Downloads or Music or Pictures or Video.

I always change the location of these directories to a separate partition or drive. In the event that the operating system crashes or gets corrupted, my files will be intact on the other drive, and recoverable.