Update SystemService instantiation docs

Extend the documentation for `startService*` calls to make it clear
which variant should be used, particularly those that rely on
reflection.

Also group service class names together based on how they are defined
and referenced from one of:
 * Custom jars in SYSTEMSERVERCLASSPATH
 * APEX jars in SYSTEMSERVERCLASSPATH
 * Standalone (APEX or othewrise) jars in STANDALONE_SYSTEMSERVER_JARS

All service classes in services.jar should be referenced explicitly.
This is a no-op change.

A follow-up change will add additional tests/safeguards for new service
references to ensure they're using the appropriate overload.

Bug: 203143243
Test: m + presubmit
Change-Id: If8b41f17b63a711127e5042ccec5dbb59f88393a