Add a sentence clarifying id-persistence to all identifiers.

[jelmerderonde]

This PR adds a sentence to all the *Id descriptions clarifying that identifiers should be persistent uuid's, always pointing to the same resource.

This information was already present in the documentation: https://openonderwijsapi.nl/#/technical/identifiers.

This PR fixes #123

[hamrt]

Hi Jelmer,

I would suggest to broaden this, and also add information about uuid to the following files:
v5/enumerations/codeType.yaml
v5/schemas/EducationSpecificationId.yaml

[jelmerderonde]

@hamrt I've made the changes you requested. Not all codes in the codeType enumeration are necessarily persistent (since they are not all part of the OOAPI spec, we have nothing to say about their persistence), so I have only added the sentence to some of the items.