Fix re-export docs (may depend on rustdoc fixes)

[sffc]

Previous issues: https://github.com/unicode-org/icu4x/issues/6069, https://github.com/unicode-org/icu4x/pull/6104, https://github.com/unicode-org/icu4x/issues/5915

We previously agreed to add docs to our re-exported input and preference types, the ones in icu_component::input and icu_component::preferences, that indicate that they are re-exports.

However, the resulting docs don't look the same in icu_component and icu::component. Here's an example:

https://docs.rs/icu_datetime/latest/icu_datetime/preferences/index.html

https://docs.rs/icu/latest/icu/datetime/preferences/index.html

Our re-export disclaimer is in icu_datetime::preferences but not icu::datetime::preferences.