Improve notes on using embedded objects vs. references

[jakubklimek]

Problem statement
Properties listed for supporting classes like Standard are probably meant to be used when an IRI of the instance of the class cannot be determined and such an instance needs to be described by these properties as an embedded object instead. When such an IRI is known, there should be no need to use the descriptive properties.

However, this approach is not explicitly described anywhere in GeoDCAT-AP, causing confusion, like in #109 .

Proposal
Explicitly mention that the properties are to be used with and embedded object when IRI of the instance of the class is unknown, which may be result of a mapping from a source which does not containt them.

[jakubklimek]

Lifting to Style Guide discussion proposed during the webinar. Issue postponed.

[jakubklimek]

Related issue SEMICeu/LDES-DCAT-AP-feeds#17