[Revise subclass] Noonan syndrome is not always a cardiogenetic syndrome

[pnrobinson]

Mondo term (ID and Label)
https://www.ebi.ac.uk/ols4/ontologies/mondo/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FMONDO_0018997

Suggested revision and reasons

@cmungall @mellybelly
There are lots of things like this in Mondo -- sometimes patients with Noonan have cardio manifestations, but not always. Therefore, it seems incorrect to call Noonan a cardiogenetic syndrome. Could we please write the rules for this sort of thing in the Mondo documentation? We have something about "relations" here https://obofoundry.org/principles/fp-007-relations.html but this does not really help to decide whether we think this is correct or incorrect -- we need to do so to be consistent!