Document issues with serializing a value as `None`

[mwchase]

I'm following along with a game-making tutorial, and when the tutorial plugged shelve for saving, I opted for camel instead. I ran across some issues that seem to be inherited from PyYaml, so they should be doc bugs, probably.

This issue is, while it's safe to use None as a value in containers, returning None directly from a dumper (because there was no state to serialize, beyond the type of the value) gave me some obscure errors from within PyYaml.