8024776: Max/MinHeapFreeRatio descriptions should be more precise

Summary: Descriptions for Max/MinHeapFreeRatio updated
Reviewed-by: ehelin, jmasa

src/share/vm/runtime/globals.hpp

@@ -3080,10 +3080,14 @@ class CommandLineFlags {
           "class pointers are used")                                        \
                                                                             \
   product(uintx, MinHeapFreeRatio,    40,                                   \
-          "The minimum percentage of heap free after GC to avoid expansion")\
+          "The minimum percentage of heap free after GC to avoid expansion."\
+          " For most GCs this applies to the old generation. In G1 it"      \
+          " applies to the whole heap. Not supported by ParallelGC.")       \
                                                                             \
   product(uintx, MaxHeapFreeRatio,    70,                                   \
-          "The maximum percentage of heap free after GC to avoid shrinking")\
+          "The maximum percentage of heap free after GC to avoid shrinking."\
+          " For most GCs this applies to the old generation. In G1 it"      \
+          " applies to the whole heap. Not supported by ParallelGC.")       \
                                                                             \
   product(intx, SoftRefLRUPolicyMSPerMB, 1000,                              \
           "Number of milliseconds per MB of free space in the heap")        \