[📚] Document query rules

[Lyokone]

This issue is for tracking the various query "edge-cases" that are probably worth documenting

The following is a non-exhaustive list of points to cover:

.orderBy().startAt().orderBy()
The issue: orderBy cannot be used if startAt was already specified
collection.startAt()
The issue: startAt requires an orderBy query
where('foo', >=).where('bar', >=)
The issue: when using certain where operators, subsequent where usages must be applied to the same property
This other list contains ODM-specific points:

orderByA().orderByB(startAt: )

copied from: firebase/flutterfire#9165