Clarify that Ninja is required for the default build experience. (dot…

…net#50809)

docs/workflow/building/coreclr/README.md

@@ -23,11 +23,13 @@ CoreCLR also supports a 'checked' build type which has asserts enabled like 'deb
 ./build.sh -subset clr -configuration checked
 ```
 
-If you want to use Ninja to drive the native build instead of Visual Studio MSBuild (on Windows) or Make (on non-Windows), you can pass the `-ninja` flag to the build script as follows:
+If you want to use Ninja to drive the native build instead of Make on non-Windows platforms, you can pass the `-ninja` flag to the build script as follows:
 ```
 ./build.cmd -subset clr -ninja
 ```
 
+If you want to use Visual Studio's MSBuild to drive the native build on Windows, you can pass the `-msbuild` flag to the build script similarly to the `-ninja` flag.
+
 We recommend using Ninja for building the project on Windows since it more efficiently uses the build machine's resources for the native runtime build in comparison to Visual Studio's MSBuild.
 
 To pass extra compiler/linker flags to the coreclr build, set the environment variables `EXTRA_CFLAGS`, `EXTRA_CXXFLAGS` and `EXTRA_LDFLAGS` as needed. Don't set `CFLAGS`/`CXXFLAGS`/`LDFLAGS` directly as that might lead to configure-time tests failing.

docs/workflow/requirements/windows-requirements.md

@@ -44,7 +44,7 @@ The dotnet/runtime repository requires at least Visual Studio 2019 16.6.
 
 The dotnet/runtime repository recommends using CMake 3.16.4 or newer, but it may work with CMake 3.15.5.
 
-## Ninja (optional)
+## Ninja
 
 - Install Ninja in one of the two following ways
   - [Download the executable](https://github.com/ninja-build/ninja/releases) and add its location to [the Default PATH variable](#adding-to-the-default-path-variable).