8221267: Document the jdk.net.URLClassPath.showIgnoredClassPathEntries system property

Reviewed-by: alanb, bpb, lancea

src/java.base/share/classes/java/lang/ClassLoader.java

@@ -1893,6 +1893,16 @@ public abstract class ClassLoader {
      * otherwise, if unnamed, it will set the class path to the current
      * working directory.
      *
+     * <p> JAR files on the class path may contain a {@code Class-Path} manifest
+     * attribute to specify dependent JAR files to be included in the class path.
+     * {@code Class-Path} entries must meet certain conditions for validity (see
+     * the <a href="{@docRoot}/../specs/jar/jar.html#class-path-attribute">
+     * JAR File Specification</a> for details).  Invalid {@code Class-Path}
+     * entries are ignored.  For debugging purposes, ignored entries can be
+     * printed to the console if the
+     * {@systemProperty jdk.net.URLClassPath.showIgnoredClassPathEntries} system
+     * property is set to {@code true}.
+     *
      * @return  The system {@code ClassLoader}
      *
      * @throws  SecurityException