manual: clarify assumptions around `~` and typical `R_USER` location on Windows

[achubaty]

Some Windows users may have their corporate machines set up to have their home directories pointing to e.g. a One Drive location rather than the typical C:/Users/username/Documents.

Explain how these users can emulate the more typical set up by setting the R_USER environment variable, and highlight that project paths etc. used for set up on Windows assume the typical home directory setup.