docs: add spec docs and todo for OP

[mattsse]

adds an excerpt from the spec, this is equivalent to legacy engine check

reth/crates/consensus/beacon/src/engine/mod.rs

Lines 459 to 479 in 72c0e65

	// On Optimism, the proposers are allowed to reorg their own chain at will.
	#[cfg(feature = "optimism")]
	if self.blockchain.chain_spec().is_optimism() {
	debug!(
	target: "consensus::engine",
	fcu_head_num=?header.number,
	current_head_num=?head.number,
	"[Optimism] Allowing beacon reorg to old head"
	);
	return true
	}
	// 2. Client software MAY skip an update of the forkchoice state and MUST NOT begin a
	// payload build process if `forkchoiceState.headBlockHash` references a `VALID` ancestor
	// of the head of canonical chain, i.e. the ancestor passed payload validation process
	// and deemed `VALID`. In the case of such an event, client software MUST return
	// `{payloadStatus: {status: VALID, latestValidHash: forkchoiceState.headBlockHash,
	// validationError: null}, payloadId: null}`
	if head != &header.num_hash() {
	attrs.take();
	}

for op we still need to trigger a payload job and perhaps even force canonicalize the head