document atomic operations for Firebase

[halatmit]

Make sure the Firebase component documentation include the atomic operations.

Also up the document at:

https://docs.google.com/document/d/17OGJF8VwbOYkXCual8I1JIBLCHhnCcgHj6N_Cfnlb4A/edit#heading=h.37rrxtruvui0

To talk about the issue with atomic operations and how to deal with this and include examples.

[jisqyv]

@halatmit I'm also going to write a blog post explaining how to use these new features.

[halatmit]

@jisqyv Good, but I also want to include that info in the other document, as part of the cloud variable documentation. You don't have to do that. Whoever does it can use what you put in the blog post.