Build: Some build cleanup, and documentation regarding static linking…

… to _Java9Impl/_Java16Impl using the multi-release JAR format.

README.md

@@ -137,6 +137,11 @@ Reproducible builds: If the resulting `freemarker.jar` is not identical with the
 in the `.buildinfo` file packed into the official source distribution, and also into the Maven "sources" artifact! At
 least with identical Java versions, the resulting `freemarker.jar` meant to match exactly.
 
+Note on trying things out with an ad-hoc class that has `main` method: Don't do that, instead write it as a JUnit test.
+FreeMarker needs to be loaded from `freemarker.jar` that contains the `META-INF/versions` directory (as per JEP 238,
+Multi-Release JAR Files). If you run a test, Gradle ensures that you have an up-to-date `freemarker.jar`, and
+that it's in the classpath. Without that, FreeMarker will behave as if you are on a lower Java version.
+
 
 ### Maven-related build tasks
 

buildSrc/src/main/kotlin/freemarker/build/FreemarkerRootExtension.kt

@@ -230,8 +230,6 @@ class FreemarkerModuleDef internal constructor(
                     .toString()
                 systemProperty("freemarker.test.resourcesDir", resourcesDestDir)
 
-                testClassesDirs = sources.output.classesDirs
-
                 // We have to build the jar and depend on that, because we depend on "JEP 238: Multi-Release JAR Files",
                 // which puts some classes into the jar under META-INF\versions\$javaVersion.
                 val jarClasspath = project.objects.fileCollection()

freemarker-manual/src/main/docgen/en_US/book.xml

@@ -30149,7 +30149,14 @@ TemplateModel x = env.getVariable("x");  // get variable x</programlisting>
               <literal>16</literal>, instead of in the usual location together
               with Java 8 byte code class files. This was needed as some
               tools, like Maven Enforcer plugin, have seen them as problems
-              otherwise.</para>
+              otherwise. Also, now we don't look for the
+              <literal>_Java<replaceable>X</replaceable>Impl</literal> classes
+              with <literal>Class.forName(String)</literal>, instead we just
+              refer them normally, as now Java will link to the proper variant
+              according the multi-release JAR format. This can help if some
+              environment doesn't support class loading on runtime (however,
+              then you still will have problems with other optional
+              features).</para>
             </listitem>
           </itemizedlist>
         </section>