Clarify discrepancies between code and package documentation regarding Epoch system

Comment regarding ChannelHeader structure inside common/common.proto implies that the Epoch system is already implemented
This comment is later used as part of the package's documentation:
https://hyperledger.github.io/fabric-protos/protos.html#common-ChannelHeader and https://pkg.go.dev/github.com/hyperledger/fabric-protos-go/common#ChannelHeader.
However, go SDK sets the Epoch to a hard coded value of 0 and contains TODO markers, e.g. in this file: https://github.com/hyperledger/fabric-sdk-go/blob/main/internal/github.com/hyperledger/fabric/protoutil/proputils.go.
This can be quite misleading and at least a warning in the documentation would be appreciated.