Document the changes regarding 6u18 VM NewRatio default value in more details

The default value for the parameter -XX:NewRatio was changed in
Java SE 6 Update 18.

This was briefly documented in the release notes of Java SE 6 Update 18:

 http://www.oracle.com/technetwork/java/javase/6u18-142093.html
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Changes in 1.6.0_18 (6u18)
[ ... ]
Performance Improvements
[ ... ]
 The maximum amount of space allocated to the young generation
  is one third of the total heap size.
[ ... ]
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 The suggestion is to especially document the implications of this change
 in more details:
 - the size of the NewGen heap is increased at the cost of the size of the
   OldGen heap
 - Applications relying on the default parameter values are likely to run
   OutOfMemory
 - The effect can be easily cured by either setting the NewRatio or the
   Xmn, or the NewSize/MaxNewSize parameters explicitly for the application.