The first note is misleading, and should probably be combined with the second to be something more like:

This setting controls the global virtual environment storage path. It most likely will not be useful at the local level. To store virtual environments in the project root, see virutualenvs.path.

"unpredictable behavior" is true as there is currently a bug with a relative path, but it makes more sense to fix that bug than it does to add nonspecific documentation warnings unlikely to be removed when the bug is fixed.