Update instructions for producing official MSW binaries

Simplify the officialbuild.bat script by only checking for "vcXX" instead of
accepting both them and "vsYYYY".

Also add instructions for using it in the corresponding HOWTO.

Closes #17514.

docs/contributing/how-to-release.md

@@ -123,3 +123,39 @@ in the generated files too.
 
 Update the definition of the stable and release branches in
 `build/buildbot/config/include/defs.xml` after a minor version change.
+
+## MSW Visual Studio Official Builds
+
+To build official x86 and x64 shared binaries the following are prerequisites:
+ - Visual Studio 2012, 2013, 2015
+ - Windows SDK 6.1, 7.1 (required for x64 builds for Visual Studio 2008, 2010)
+ - 7z (required for packaging the files)
+ - fciv (required for generating the checksums)
+
+The VSxxxCOMNTOOLS environment variables are used to locate the tools required
+for Visual Studio 2012, 2013 and 2015. There are no Microsoft defined variables
+for the SDKs used for Visual Studio 2008 and 2010. The build will look for the
+following environment variables for the Visual Studio 2008 and 2010 SDK tools:
+
+WINDOWS61SDK
+WINDOWS71SDK
+
+If either of these are blank they are set to the default install location.
+
+To build binaries for a single compiler, open a command prompt (for Visual
+Studio 2008 only an SDK 6.1 developer's command prompt must be used),
+cd to the build\msw\tools\msvs folder and run the batch file 'buildofficial'
+with the vcXXX version number:
+
+Visual Studio 2008  vc90
+Visual Studio 2010  vc100
+Visual Studio 2012  vc110
+Visual Studio 2014  vc120
+Visual Studio 2015  vc140
+
+This will build all of the x86 and x64 binaries for the selected compiler version,
+package them in 7z files and calculate the checksums. The 7z files and the
+checksums are output to the build\msw\packages folder.
+
+All of the compiler packages can be built at the same time by executing the
+build\msw\tools\buildall.bat file (including Visual Studio 2000).