8182482: Module System spec updates

[Alan Bateman]

We have two javadoc/spec updates that I'd like to get into the JDK 9 
Initial Release Candidate that is scheduled for this week.

The spec updates are for two issues:

1. ServiceLoader: The API spec has been updated significantly to support 
modules but it needs another round of update to do clean-up to get it 
more readable and consistent, and also to align it with the JLS.  Most 
of reorganization and re-wording has been proposed by Alex. Joe Darcy 
has also proposed a few adjustments.

2. Upgradable modules aren't specified anywhere. Java SE will designate 
a number of standard modules as upgradeable but we don't have anywhere 
in the docs to link to that or describe how the upgraded versions are 
used in preference to the modules built into the environment.

The webrev with the proposed (docs only, no implementation) changes is here:
   http://cr.openjdk.java.net/~alanb/8182482/webrev/index.html

The ServiceLoader diffs are hard to read. It might be easier to read the 
generated javadoc:
http://cr.openjdk.java.net/~alanb/8182482/docs/java/util/ServiceLoader.html

-Alan