[referenceFinder] refine a javadoc for IResourceAccess

Signed-off-by: Anton Kosiakov <anton.kosyakov@typefox.io>

org.eclipse.xtext/src/org/eclipse/xtext/findReferences/IReferenceFinder.java

@@ -44,6 +44,8 @@ public interface IReferenceFinder {
 	/**
 	 * Provides safe read access to a resource set for searching local references or references in a demand-created or
 	 * shared resource set.
+	 * 
+	 * Return `null` if a resource set cannot be provided for the given target URI.
 	 */
 	interface IResourceAccess {
 		<R> R readOnly(URI targetURI, IUnitOfWork<R, ResourceSet> work);