Improve documentation for alternate Java runtime

We should provide a few more details about the optional Java Runtime Environment (JRE) feature:

• When you specify the path to the alternate Java runtime during cluster creation, is that path applied to all current and future nodes in the cluster? And does this mean that all nodes that use Java will attempt to use the alternate runtime?
• What happens if the alternate Java runtime cannot be found at the specified path? (e.g. is there a warning and/or does Couchbase fall back to the installer-supplied JDK?)
• Is the path meant to point to an installation path, or a package? (i.e., does the alternate Java runtime need to be manually installed , or can you just point to a package?)
• Can you specify an alternate Java runtime after the cluster is created? If yes, how?