XmlNodeBuilder insufficiently documented

### Affected version

HEAD

### Bug description

There's some surprising behavior in XmlNodeBuidler around reading multiple documents from one stream as I found in the tests:

impl/maven-xml/src/test/java/org/apache/maven/internal/xml/XmlNodeBuilderTest.java

This is apparently deliberate. It's not necessarily wrong, but it needs to be thoroughly documented, not just tested.