[DOCS] improve op-conductor docs

[sbvegan]

Brief Description of the Docs Request

The --rollup-rpc section needs an additional important detail.

Description of the Documentation You'd Like

The op-node that is behind the op-conductor must have the safe head db enabled so it won’t be stateless in this config. Both op-node and op-geth need data back to at least when the games start.

Links to Related Pages

https://docs.optimism.io/builders/chain-operators/tools/op-challenger#--rollup-rpc

Additional Context

This is the op-node help information:

--safedb.path value                                                    ($OP_NODE_SAFEDB_PATH)
          File path used to persist safe head update data. Disabled if not set.