Bring "Messages" section in spec up to date

[Dominik1999]

https://raiden-network-specification.readthedocs.io/en/latest/messaging.html seems not to be up to date.

We will need to

• [x] see if the messages listed there are in the code anymore (withdraw messages seem not to be up to date)
• [x] adjust the example messages
• [ ] make it a good read

[Dominik1999]

good enough for now