Issue #15414: Clean and correct the os.path.join docs.

In particular, correctly describe the behavior of ntpath.join.

Based on a patch by Dave Sawyer.
Closes #15414: Merge with 3.4
Issue #22604: Fix assertion error in debug mode when dividing a complex number by (nan+0j).
Issue #22604: Fix assertion error in debug mode when dividing a complex number by (nan+0j).