Fix unnecessary uses of {@docRoot} in serviceability APIs

Now that we generate a single unified docs bundle, instead of lots of smaller ones, we no longer need {@docRoot}/.. to link between the bundles.

[Aside: we didn't need them before: -link would have been a better solution!]

There are a few such references that can be replaced by using @see and {@link}.

Module -- Target of link using {@docRoot}

jdk.jconsole -- api/java/util/ServiceLoader.html
jdk.jdi -- api/java/lang/ThreadGroup.html
jdk.management -- api/java/lang/management/ManagementFactory.html#getPlatformMBeanServer()
jdk.management -- api/java/lang/management/package-summary.html
jdk.management -- api/javax/management/MBeanInfo.html#info-changed