Per discussion in #84326. For trait implementations, this was
misleading: the items actually do have documentation (but it comes from
the trait definition).
For both trait implementations and trait implementors, this was
redundant: in both of those cases, the items are default-hidden by
different toggle at the level above.
Update tests: Remove XPath selectors that over-specified on details tag,
in cases that weren't testing toggles. Add an explicit test for toggles
on methods. Rename item-hide-threshold to toggle-item-contents for
consistency.
