doc: clarify the prerequisites for building with VS2017

* "Edition" is ambiguous. For example, Visual Studio Team Explorer 2017
  is listed under "Visual Studio 2017", but Build Tools for Visual
  Studio 2017 is listed under "Other Tools and Frameworks".
* The listed required components are insufficient. VS2017 also needs
  "Visual Studio C++ core features", and Build Tools also needs
  "Visual C++ Build Tools core features".
* Installing the workload with the default optional components takes up
  only about 1GB more space than the minimal set of components, but
  saves scrolling through the long list of individual components.

PR-URL: #16903
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

BUILDING.md

@@ -199,9 +199,11 @@ Prerequisites:
   * [Visual Studio 2015 Update 3](https://www.visualstudio.com/), all editions
     including the Community edition (remember to select
     "Common Tools for Visual C++ 2015" feature during installation).
-  * [Visual Studio 2017](https://www.visualstudio.com/downloads/), any edition (including the Build Tools SKU).
-    **Required Components:** "MSbuild", "VC++ 2017 v141 toolset" and at least one of the Windows SDKs.
-    *Note*: For "Windows 10 SDK (10.0.15063.0)" only the "Desktop C++ x86 and x64" flavor is required.
+  * The "Desktop development with C++" workload from
+    [Visual Studio 2017](https://www.visualstudio.com/downloads/) or the
+    "Visual C++ build tools" workload from the
+    [Build Tools](https://www.visualstudio.com/downloads/#build-tools-for-visual-studio-2017),
+    with the default optional components.
 * Basic Unix tools required for some tests,
   [Git for Windows](http://git-scm.com/download/win) includes Git Bash
   and tools which can be included in the global `PATH`.