improve: [N07] Document that payable functions cannot be included in MultiCaller functions

[nicholaspai]

Issue:

• [N07] payable multicall function disallows msg.value
  • The MultiCaller contract is inherited by the HubPool and SpokePool contracts. It provides the public
    multiCall function that facilitates calling multiple methods within the same contract with only a single
    call.
  • However, although it is designated as a payable function, it disallows any calls that send ETH, ie where
    msg.value is not zero.
  • This effectively makes the payable designation moot and the contradictory indications could lead to
    confusion.
  • In the context of the HubPool, specifically, relays destined for chains where ETH is required and where
    a call to loadEthForL2Calls is therefore necessary, will not be multi-callable.
    Consider either explicitly noting this limitation, or removing both the require statement and the
    payable designation.

Resolution:

• Document payable functions that would naturally be included in multicall bundles, such as loadEthForL2Calls, this issue.

[mrice32]

LGTM!