clarify use of additionalMetadata element

Clarified how the 'additionalMetadata' element is used, as per Bryce's comment.

docs/eml-semantic-annotations-primer.md

@@ -220,7 +220,7 @@ The third annotation also has as its subject the `creator` element that has the
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 ### Pattern for the `additionalMetadata` element annotations
-Semantic annotations may also be inserted in the `additionalMetadata` element that is nested under the `eml` root element. This type of semantic annotation has a `describes` element and a `metadata` element containing the annotation. The `metadata` element has an `annotation` element. The content of the `describes` element is the id of the *subject* of the semantic annotation. The subject is the element containing the id. Contained within the `annotation` element are `propertyURI` and `valueURI` elements. The `propertyURI` is the *object property* or *data property* and the `valueURI` is the *object* of the annotation. *The URIs should ideally point to terms in controlled vocabularies*. Each `propertyURI` and `valueURI` element can have a `label` attribute that displays a label associated with each URI. Labels are intended to provide a more readable format for users and may be displayed in application interfaces. *It is recommended that labels are populated with values from the preferred labels field (skos:prefLabel) or label field (rdfs:label) from a controlled vocabulary.* 
+Semantic annotations may also be inserted in the `additionalMetadata` element that is nested under the `eml` root element. This element is a container for supplemental non-EML metadata that pertains to the subject of interest. Only metadata that is XML-based can be inserted here. Each `additionalMetadata` element semantic annotation has `describes` elements and `metadata` elements for annotations, with each`metadata` element containing at least one `annotation` element. The content of the `describes` element is the id of the *subject* of the semantic annotation. The subject is the element containing the id. Contained within the `annotation` element are `propertyURI` and `valueURI` elements. The `propertyURI` is the *object property* or *data property* and the `valueURI` is the *object* of the annotation. *The URIs should ideally point to terms in controlled vocabularies*. Each `propertyURI` and `valueURI` element can have a `label` attribute that displays a label associated with each URI. Labels are intended to provide a more readable format for users and may be displayed in application interfaces. *It is recommended that labels are populated with values from the preferred labels field (skos:prefLabel) or label field (rdfs:label) from a controlled vocabulary.* 
 
 Multiple `annotation` elements may be embedded in the same `metadata` element to assert multiple semantic statements about the same subject. Annotating different subjects requires using additional `describes` elements.
 ```xml