fixup! Fix Ignored RootNS

- add doc comment
- update test names

DAFFODIL-2947

daffodil-core/src/main/scala/org/apache/daffodil/core/dsom/SchemaSet.scala

@@ -362,6 +362,11 @@ final class SchemaSet private (
         if (possibleRoots.length == 1) {
           possibleRoots.head
         } else {
+          // we're here because it's ambiguous what root element to return, and
+          // no namespace URI was provided. So we assume not specifying a namespace
+          // URI may mean the intention was to choose the root that it is in No Namespace.
+          // If there is no such, then the diagnostic will encourage user to specify
+          // the namespace explicitly.
           val qn = RefQName(None, rootElementName, NoNamespace)
           findRootElement(qn)
         }

daffodil-tdml-processor/src/test/scala/org/apache/daffodil/processor/tdml/TestTDMLRunner2.scala

@@ -609,7 +609,7 @@ abc # a comment
   @Test def test_apos_test2(): Unit = { runner.runOneTest("apos_test2") }
 
   // DFDL-2947
-  @Test def test_multipleRootCandidates1(): Unit = {
+  @Test def test_rootNSSpecifiesNoNamespaceRoot(): Unit = {
     val tdmlTestSuite =
       <tdml:testSuite suiteName="theSuiteName" xmlns:ex={example} xmlns:tdml={tdml} xmlns:dfdl={
         dfdl
@@ -628,7 +628,7 @@ abc # a comment
     runner.runOneTest("test1")
   }
 
-  @Test def test_multipleRootCandidates2(): Unit = {
+  @Test def test_rootNSSpecifiesNamespaceRoot(): Unit = {
     val tdmlTestSuite =
       <tdml:testSuite suiteName="theSuiteName" xmlns:ex={example} xmlns:tdml={tdml} xmlns:dfdl={
         dfdl