Currently https://unicode-org.github.io/icu4x/rustdoc/icu_calendar/cal/struct.IslamicObservational.html says is "Islamic Observational Calendar (Default)".

The docs should explain at least:

• That "observational" means an astronomical simulation of moon observability from a reference location on Earth.
• That the reference location is Cairo and why it is Cairo. (The code comment points to Reingold's code, which says Cairo is a "sample location".)
• Why this is "(Default)".
• What use cases this variant is for relative to the other three variants.