+   - If the name of an existing script is passed in ``argv[0]``, its absolute
+     path is prepended to :data:`sys.path`

Absolute path to the directory where script is located.  And I believe there's no absolute path guarantee for platforms without realpath / GetFullPathName.

Should the documentation also give some guidance to those that embed python and don't want to start using SetArgvEx right away and break compatibility with older python versions?  Something like:

If you're embedding python in your application, using SetArgv and don't want modified sys.path, call PyRun_SimpleString("sys.path.pop(0)\n"); after SysArgv to unconditionally drop the first sys.path argument added by SetArgv.