Max/MinHeapFreeRatio descriptions should be more precise

Description of MaxHeapFreeRatio and MinHeapFreeRatio states that these flags applies to a whole heap. However, it is true only for G1, while of Serial and CMS GC these flags applies only to old gen and Parallel GC does not support these flags at all.

Description of these options should be updated to reflect the fact, that Min/MaxHeapFreeRatio applies to individual regions, but not to a whole heap.

It is crucial, because various documents on VM options almost always use info from globals.hpp to describe various options (for example http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html).

So if whole heap free ratio will not fit into Min/MaxHeapFreeRatio limits, user may treat it as an issue, while it is not true just because these limits are not applicable to young gen.