More precise document on os.path.normpath()

[Peng Yu]

After I tried os.path.normpath(), it is clear that the function
doesn't return the trailing '/', if the path is a directory. But this
fact is not documented. Should this be documented in future release of
python.

Also, I found the documentation of some functions in os.path are not
clear. I have to try them in order to understand them for corner
cases. I'm wondering if I can join the people who maintain the
document to help improve it.