Clarify some details on RGB-LN docs

[St333p]

This PR improves the "RGB over Lighting Network" chapter in the following ways:

• The first couple of commitments is now representative of the simple case, without a push amount at channel creation
• The description of what happens if one of the parties broadcasts an old state is now after the second couple of commitments, so that we have an old state to begin with. Roles were also inverted, since Alice is the one having an economic incentive to attempt such an attack.
• A final comment was added to point out why RGB-LN payments also need to transfer a certain amount of sats as well