[Rd] Feature request: Change default library path on Windows

[Steve Haroz]

> I think the benefits of fixing the default behavior easily outweigh the
> backward-compatibility and "there is a documented workaround"
> arguments.

Exactly. This is an accessibility issue and a serious barrier for some
beginners. It's possible to work around the error message, but users
should not have to do a workaround the very first time they install a
couple libraries.
Many users synching with one of several popular cloud services and
many users whose Documents folder is managed by a Network IT policy
will run into this issue.

Moving the R libraries from USER/Documents/R to USER/R will help avoid
"attrition" of beginners.

-Steve