Add brief to gc.heap.generation members

Author: stevejgordon

model/registry/dotnet.yaml

@@ -11,18 +11,23 @@ groups:
           members:
             - id: gen0
               value: 'gen0'
+              brief: "Generation 0"
               stability: experimental
             - id: gen1
               value: 'gen1'
+              brief: "Generation 1"
               stability: experimental
             - id: gen2
               value: 'gen2'
+              brief: "Generation 2"
               stability: experimental
             - id: loh
               value: 'loh'
+              brief: "Large Object Heap"
               stability: experimental
             - id: poh
               value: 'poh'
+              brief: "Pinned Object Heap"
               stability: experimental
         brief: Name of the garbage collector managed heap generation.
-        examples: ["gen0", "gen1", "gen2", "loh", "poh"]
+        examples: ["gen0", "gen1", "gen2"]