Add helpful message about pooled classes

Hopefully this will make fewer people confused at all of the null properties. The string won't wrap nicely so I tried to keep it pithy:

https://s3.amazonaws.com/uploads.hipchat.com/6574/26709/v8vzKAC784QMkX2/upload.png
Merge pull request #1664 from spicyj/warn-pooling

Add helpful message about pooled classes