Global and Temporary global variables don't behave consistently with how they are described

[gerzytet]

When you assign something to a global variable then pull it back out, it results in a copy of the object. If you do the same with a temporary global, you just get a reference to the original. This is confusing for people who don't know the difference, especially because this difference isn't documented anywhere. I think we should do 2 things:

1. Document it, obviously.
2. Rename "Temporary Global Variable" to "Shared Variable". It's a more accurate name. We can still call it a Temporary Global in the code, it would just be a documentation change.

Related issue: #259