You asked for it, we created it. The Bro language cheat sheet is now available from our community presence at github: https://github.com/Bro/cheat-sheet. This document describes the scripting language on a single page, and also provides a solid reference of the most important built-in functions. Print it and place it next to your cup of coffee, as you are now equipped to write the next APT detector in Bro.
We provide the source code of the cheat sheet, which comes with a Creative Attribution-NonCommercial-ShareAlike license. This means you can adapt and redistribute it for non-commercial purposes as long as the attribution remains intact. If you feel that the document could be improved, we encourage you to use the proven github model of forking, udpating, and creating a pull request.
To whet your appetite, check out the two screen shots of the first two pages below. Enjoy!