firewood: document how to configure it

[rkuris]

Describe the bug
@Elvis339 reported:

Cache strategy was hardcoded on the Rust side (ffi/src/handle.rs) per Ron's suggestion, bypassing the Go config chain that was broken in the initial run. The cache hit differences across runs confirm each strategy is genuinely distinct.

It should be possible to configure the cache strategy from the go side. We could use some clearer documentation on this.

To Reproduce
Running the re-execution benchmark with only this changed does not seem to affect the run

I'm fairly familiar with re-execution test, the only thing it sets is snapshot cache:

avalanchego/tests/reexecute/c/vm_reexecute.go

Line 71 in a69e790

"snapshot-cache": 0,

Expected behavior
Would like to see exactly where to change the config settings for firewood

Additional context
There's a bunch more info here: https://docs.google.com/document/d/1NacVT1sllwJrv0cIxeHx56z8i-0uJDQGxgFcxK9Q50c/edit?tab=t.tsu2uf3h1ynt