__* name mangling documentation

[Michel Pelletier]

http://www.python.org/doc/current/ref/id-classes.html

sort of left me hanging on why __ name mangling exists and why it would be 
used.  I'm curious because I'm looking at some code right now that uses it in 
a somewhat tricky way and I want to know why.  If someone would be so kind to 
enlighten me on the reasons for __ name mangling, I will submit a 
documentation patch for the above page that explains it.

Thanks!

-Michel