Document `RwLockWriteGuard::downgrade()` guarantees

[nazar-pc]

parking_lot has important details about guarantees provided by RwLockWriteGuard::downgrade(): https://docs.rs/parking_lot/0.12.4/parking_lot/type.RwLockWriteGuard.html#method.downgrade

It'd be very helpful to know what guarantees are provided by RwLockWriteGuard::downgrade() in async-lock.

For example, I need the following:

1. downgrade is atomic and no writers can acquire lock in the meantime
2. no currently waiting writers prevent any pending or future read locks from being acquired

parking_lot guarantees 1, but does not fully guarantee 2 (though it doesn't say it will never happen either).

[nazar-pc]

I'd appreciate if any of the maintainers could look a this issue and document the guarantees. It is very important for library users.