bpo-6689: os.path.commonpath raises ValueError for different drives isn't documented...

bpo-6689: os.path.commonpath raises ValueError for different drives isn't documented (GH-14045)

It would raise ValueError("Paths don't have the same drive") if the paths on different drivers, which is not documented.

os.path.commonpath raises ValueError when the *paths* are in different drivers, but it is not documented.
Update the document according @Windsooon 's suggestion.
It actually raise ValueError according line 355 of [test of path](https://github.com/python/cpython/blob/master/Lib/test/test_ntpath.py)

https://bugs.python.org/issue6689