Make documentation around iframe permissions more generic

https://github.com/hypothesis/client/pull/6156 is introducing some new documentation about the requirement of allowing `clipboard-write` when loading Hypothesis in an iframe, how to allow that permissions and what happens if you don't do it.

However, as mentioned in https://github.com/hypothesis/client/pull/6156#discussion_r1472571713, there are other potential permissions that might be needed, so it would be good to rename the section to just `Permissions when loaded in an iframe`, and document all of them.