Replace ad-hoc detaches with spec point building blocks

[lawrence-forooghian]

(Split from #66.)

There are some spec points where the wording led me to believe that I had to perform some ad-hoc channel detachments, but they actually were meant to refer to performing a procedure defined in another spec point. Update implementation and tests accordingly:

• CHA-RL2h3 now clarifies that the whole CHA-RL2f detachment cycle should be retried
• CHA-RL3f now clarifies that the whole CHA-RL3d detach sequence should be retried
• In CHA-RL4b5 the "any non-detached contributors are detached" has been replaced by "a CHA-RL2f detach procedure is performed"
• CHA-RL1h6 means to retry the whole DETACH operation, not just the channel detach

┆Issue is synchronized with this Jira Task by Unito