Explain payment/paying channels

[oberstet]

One feedback we got from early app developers is that the terms "payment channel" and "paying channel" are confusing.

We should rename those as in:

• Payment channel => Buyer (delegate) channel
• Paying channel => Seller (delegate) channel

[oberstet]

won't fix (we won't rename): too much work without gain.

instead we should explain the terms in the docs:

• Payment channel => off-chain channel for a buyer delegate
• Paying channel => off-chain channel for a seller delegate