Clarify enr content value format about enr

[AgeManning]

To be generic across all types we store the underlying data as bytes and convert to specific types later one.

The general spec indicates an RLP list of k,v: https://github.com/ethereum/devp2p/blob/master/enr.md#rlp-encoding

In principle, it should work for any object that is RLP encodable. In your test could you not just put the fork_id into add_value() and it will be encoded as every other type that is rlp encodeable and stored in the ENR.

Your suggestion would be to double rlp-encode objects right?
Currently we are just rlp-encoding the types and storing the bytes of the rlp-encoded object, which seems sensible to me.

Unless the .build() fails on normal types, it should be the case that all rlp-bytes can be decoded as bytes right?