Nuance to EnumValueAdded's criticality

[cjoudrey]

At the moment, all EnumValueAdded changes are marked as dangerous because it may break clients that do not handle a certain enum value.

This is true if you're using the enum as a field type, but if you're only using it as an argument type, adding a new enum value should be safe.

[jturkel]

Similarly should removing an enum value for enums only used in field types and not argument types be safe? In that case, the enum value should never appear in a client request so I don't think it could break anything.