Document the usage of subject object fields more clearly

[e-backmark-ericsson]

Today all event schemas contain the fields id, source and type in their subject objects, but they are not all described in the corresponding event specific md file. They are explained in the spec.md file, but that is not obvious when reading the event specific md.

It should also be clarified what fields in the subject object are mandatory and not, and what fields have default values.

The type field should maybe have the format of an enum with only one valid value, which should then also be the default value of the type.