xdoc: document when a field is required #549
Description
for example https://maven.apache.org/ref/3.9.14/maven-model/maven.html#class_project
modelVersion or groupId are required: https://github.com/apache/maven/blob/maven-3.9.14/maven-model/src/main/mdo/maven.mdo#L85
but it is not visible for now in the doc