[OpenJDK 2D-Dev] RFR: 8182610: Update specification of service providers for IIORegistry and ServiceRegistry

[Phil Race]

This is a doc-only fix to point to ServiceLoader to explain how service 
providers work
replacing the local incomplete documentation in these Image I/O classes.
The existing docs pre-date the existence of ServiceLoader which of 
course is further
updated in 9 to explain how to deliver service providers as modules.

Bug: http://cr.openjdk.java.net/~prr/8182610/
Webrev: http://cr.openjdk.java.net/~prr/8182610/
Specdiff: 
http://cr.openjdk.java.net/~prr/8182610/iiospecdiff/package-summary.html

-phil