Update some descriptions.

Author: xingwu1

specification/batch/data-plane/Microsoft.Batch/stable/2018-12-01.8.0/BatchService.json

@@ -12477,7 +12477,8 @@
         },
         "stats": {
           "$ref": "#/definitions/JobStatistics",
-          "title": "Resource usage statistics for the entire lifetime of the job. The statistics may not be immediately available. The Batch service performs periodic roll-up of statistics. The typical delay is about 30 minutes."
+          "title": "Resource usage statistics for the entire lifetime of the job.",
+          "description": "This property is populated only if the CloudJob was retrieved with an expand clause including the 'stats' attribute; otherwise it is null. The statistics may not be immediately available. The Batch service performs periodic roll-up of statistics. The typical delay is about 30 minutes."
         }
       },
       "title": "An Azure Batch job."
@@ -13401,7 +13402,8 @@
         },
         "stats": {
           "$ref": "#/definitions/PoolStatistics",
-          "title": "Utilization and resource usage statistics for the entire lifetime of the pool. The statistics may not be immediately available. The Batch service performs periodic roll-up of statistics. The typical delay is about 30 minutes."
+          "title": "Utilization and resource usage statistics for the entire lifetime of the pool.",
+          "description": "This property is populated only if the CloudPool was retrieved with an expand clause including the 'stats' attribute; otherwise it is null. The statistics may not be immediately available. The Batch service performs periodic roll-up of statistics. The typical delay is about 30 minutes."
         }
       },
       "title": "A pool in the Azure Batch service."
@@ -13710,7 +13712,7 @@
       },
       "required": [ "coordinationCommandLine" ],
       "title": "Settings which specify how to run a multi-instance task.",
-      "description": "Multi-instance tasks are commonly used to support MPI tasks."
+      "description": "Multi-instance tasks are commonly used to support MPI tasks. In the MPI case, if any of the subtasks fail (for example due to exiting with a non-zero exit code) the entire multi-instance task fails. The multi-instance task is then terminated and retried, up to its retry limit."
     },
     "TaskStatistics": {
       "properties": {