Add documentation regarding the "unspecified" architecture feature

docs/build/cmake-presets-vs.md

@@ -264,6 +264,13 @@ The `architecture.strategy` and `toolset.strategy` values tell CMake how to hand
 
 You should use `set` with IDE generators like the Visual Studio Generator. You should use `external` with command-line generators like Ninja. These values allow vendors like Visual Studio to source the required environment before CMake is invoked. For more information about the architecture and toolset fields, see the [list of Configure Presets](https://cmake.org/cmake/help/latest/manual/cmake-presets.7.html#configure-preset).
 
+If Ninja is the generator and you don't want to source an environment, you can set `architecture.strategy` to `external` and `architecture.value` to `unspecified`. You may find it useful not to source an environment if:
+
+- you use a toolset other than MSVC,
+- you use a custom toolchain, such as in embedded scenarios,
+- you don't need a specific environment to build,
+- or you use a Linux or other non-Windows environment.
+
 For a full list of IDE generators that support the architecture field, see [`CMAKE_GENERATOR_PLATFORM`](https://cmake.org/cmake/help/latest/variable/CMAKE_GENERATOR_PLATFORM.html). For a full list of IDE generators that support the toolset field, see [`CMAKE_GENERATOR_TOOLSET`](https://cmake.org/cmake/help/latest/variable/CMAKE_GENERATOR_TOOLSET.html).
 
 Use the following examples to target ARM64 with the Ninja generator, or to target Win32 (x86) with the Visual Studio 16 2019 generator: