Make documentation around iframe permissions more generic

[acelaya]

#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 #6156 (comment), 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.