Add missing namespace descriptions, and update date (dotnet#127)

Author: natke

dotnet/docfx.json

@@ -56,7 +56,7 @@
       "apiPlatform": "dotnet",
       "author": "natke",
       "ms.author": "nakersha",
-      "ms.date": "11/08/2019",
+      "ms.date": "01/02/2020",
       "uhfHeaderId": "MSDocsHeader-DotNet",
       "searchScope": [".NET"],
       "ms.topic": "managed-reference",

dotnet/xml/ns-Microsoft.Extensions.ML.xml

@@ -1,6 +1,6 @@
 <Namespace Name="Microsoft.Extensions.ML">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Utilities for extending the ML.NET API, including the prediction engine pool</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>

dotnet/xml/ns-Microsoft.ML.Vision.xml

@@ -1,6 +1,6 @@
 <Namespace Name="Microsoft.ML.Vision">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Namespace containing image classification components.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>