Update MSIG comment

[heifner]

MSIG does not use deferred transactions:

https://github.com/eosnetworkfoundation/eos-system-contracts/blob/main/contracts/eosio.msig/include/eosio.msig/eosio.msig.hpp#L90

See #56

Also update comment to indicate how delay_sec can be used to restrict time of execution of transaction.

[ericpassmore]

Ah, missed that with the deferred transaction work.