Add a controller doc about garbage collection

[clux]

Feel like i've explained GC to a good couple of handful of people at this point, so want to have a doc to point to.

[clux]

Might be a quick one for you to look at @nightkr if you have time - it's kind of just expanding on your two examples.

[nightkr]

About to head off, but I'll try to get through it tomorrow.