Give more details on differences between internalId and default id primary keys

[asishallab]

Currently, at least the "for developers" documentation only explain that if "internalId" is not specified a default primary key, named "id" is going to be generated. But there are more differences, e.g. in the GraphQL schema and mutations. In the create case the input type does not expect a value for default id primary keys, while a unique value is expected in the internalId case. Also, do default IDs work for all storages?

Please introduce a new, concise, but exhaustive "IDENTIFIER" section in which these issues are explained.