Doc fix and improvement.

Only exact version of Windows SDK works.
Clarify how to do a release build.

BUG=angleproject:2297

Change-Id: I2566b7e807a84c4edc3b457299036e67a64b5a36
Reviewed-on: https://chromium-review.googlesource.com/852553
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

doc/DevSetup.md

@@ -21,7 +21,7 @@ On Windows:
    * Put `is_clang = false` in your gn args to compile with the Microsoft Visual C++ compiler instead of clang.
    * See the [Chromium Windows build instructions](https://chromium.googlesource.com/chromium/src/+/master/docs/windows_build_instructions.md) for more info.
    * Required for the packaged Windows 10 SDK.
- * [Windows 10 Standalone SDK version 10.0.15063 or later](https://developer.microsoft.com/en-us/windows/downloads/windows-10-sdk)
+ * [Windows 10 Standalone SDK version 10.0.15063 exactly](https://developer.microsoft.com/en-us/windows/downloads/windows-10-sdk)
    * Comes with additional features that aid development, such as the Debug runtime for D3D11. Required for the D3D Compiler DLL.
  * [Cygwin's Bison, flex, and patch](https://cygwin.com/setup-x86_64.exe) (optional)
    * This is only required if you need to modify GLSL ES grammar files (`glslang.l` and `glslang.y` under `src/compiler/translator`, or `ExpressionParser.y` and `Tokenizer.l` in `src/compiler/preprocessor`).
@@ -51,10 +51,12 @@ gn gen out/Debug
 
 GN will generate ninja files by default.  To change the default build options run `gn args out/Debug`.  Some commonly used options are:
 ```
-is_debug = true
 target_cpu = "x64"  (or "x86")
-is_clang = false  (to use compiler other than clang)
+is_clang = false    (to use system default compiler instead of clang)
+is_debug = true     (enable debugging, true is the default)
 ```
+For a release build run `gn args out/Release` and set `is_debug = false`.
+
 For more information on GN run `gn help`.
 
 Ninja can be used to compile on all platforms with one of the following commands: