Improve engine and state root task docs

[mattsse]

Describe the change

the section after

reth/crates/engine/tree/src/tree/mod.rs

Line 2380 in 2ee7748

let (state_root_handle, state_root_task_config, state_root_sender, state_hook) =

is lacking useful docs/notes and is not trivial to follow for someone unfamiliar

especially

reth/crates/engine/tree/src/tree/root.rs

Line 656 in 2ee7748

fn run(mut self, sparse_trie_tx: Sender<SparseTrieUpdate>) -> StateRootResult {

which currently doesn't have any docs

TODO

• document all parties involved in StateRootTask::run and how message passing works
• annotate block insertion steps with docs

Additional context

No response