Clarify documentation

[Oliver]

Hi! Great project. Some of the docs do not make sense, though.

For example these parts in the getting started guide:

It would allow her to cheat by settling on a value less than sent one. The sender initiates channel settling by the same machinomy close command. The get the funds a receiver is expected to finish settle on her side.

There are probably some more, but not ones I have noticed as of yet. Generally, it is a bit confusing that you switch between using seller/receiver and sender/buyer in the same passage of text throughout the documentation. Settling on just one would make things a bit clearer.