Please add docs about `fire` prototypes

[raspberrypuppies]

The `fire` prototype is used for both spitter puddles and fires. It has a number of properties for merging fires, adding fuel to fires, secondary animations, and a mysterious `uses_alternative_behavior` flag. The documentation for these is sparse and I'd like to avoid hours and hours of guess and checking to reverse engineer it. Can you help me out?

For my mod, I have new kinds of spitter-like puddles. One turret creates large puddles and another creates clusters of small ones. I'd like to make sure they only merge with similar puddles (ie not acid or flamethrower fire) and have control and understanding over how they merge (ie each big puddle counts as 5x the small ones for fuel and merging). Can you please document how the merge and fuel mechanics work?

Secondary animations are another mystery. Judging by examples, it appears that the secondary animations are generally smaller than the primary one but I don't know when or why it uses the secondary one. The primary animation already has support for sprite variations so the secondary one must have some special purpose. Can you please document what these mean?

I'd like to understand what the `uses_alternative_behavior` flag does. There are a handful of properties that say "does something else when `uses_alternative_behavior` is set but it's not clear if the alternative behavior is purely cosmetic or changes the fire's game mechanics. Can you please document what these mean and say why someone might want alternative_behavior true or false?

Thank you for your help!