Should have more detail about named return values.

There was not a section about named return values, and it feels like it is a valuable and important enough thing to learn early on. If nothing else, when looking at someone else's code this may be a point of confusion.
Fixed `on` to `in` in comment.
Slight language change per pull req. comment
Hopefully slight language improvement over orig.
Merge pull request #657 from szaydel/master

Golang - lack of information about named return values in functions