Document multisig FWSS use

[rvagg]

While this is top of mind for me: if someone wants to use FWSS with a multisig (like Safe), then they have to jump through hoops but it's possible and we should find a place to document this.

FilOzone/dealbot#416 and FilOzone/dealbot#419 is where I bumped against the friction and it has two main forms:

1. Getting Filecoin Pay set up is not straight forward. Because of the need to do an EIP-2612 signature for permit() you can't call the various methods we have on Filecoin Pay, like depositWithPermitAndApproveOperator() to do the one-step setup. Unless the multisig can handle it. So You have to decompose into three separate transactions (batch in Safe is nice, but still a bit of a drama): USDFC.approve(), FilecoinPay.deposit() and FilecoinPay.setOperatorApproval(). None of these have a separate signature verification so they are easy to push through a multisig.
2. Using a multisig as the owner wallet in Synapse causes Lotus EOA dramas. This is the problem @nijoe1 has been attempting to solve @ fix(eth): allow eth_call and eth_estimateGas from contract and non-existent addresses filecoin-project/lotus#13470 and it's still not reviewed and landed (we'll get there!), so for now you get SysErrSenderInvalid from eth_calls that Synapse makes because it'll always set the wallet as the from. But you can work around this with a custom transport, which is what I did in fix: work around Lotus eth_call rejection for multisig accounts dealbot#419 which also happens to involve session keys, so it's for the readClient, but same principle applies for the client. Intercept any eth_call or eth_estimateGas and remove from from the request and pass it on, then Lotus doesn't bother you for what we do (although I don't think we run eth_estimateGas in Synapse, mostly, and maybe this trick won't work for that anyway? I haven't checked.)