MSVC-Builds.md: Mention about toolchains and targets

... when one is doing a cross-build and/or when one specifies a
particular toolchain version.  This will make things clearer to people
what the build will look for in different build scenarios.

win32/MSVC-Builds.md

@@ -24,11 +24,18 @@ dependencies:
 
 You will also need the following tools:
 *  Visual Studio 2013 or later, with C/C++ compilation support (MSVC).
-*  The Rust Compiler and tools with the msvc toolchain(s) installed, that
-   matches the architecture that is being built.  It is recommended to use
-   the `rustup` tool from https://www.rust-lang.org/ to install and
-   configure Rust, which will install Rust in `%HOMEPATH%\.cargo` by
-   default.
+*  The Rust Compiler and tools with the msvc toolchain(s) and/or target(s)
+   installed, that matches the architecture that is being built.  It is
+   recommended to use the `rustup` tool from https://www.rust-lang.org/
+   to install and configure Rust, which will install Rust in
+   `%HOMEPATH%\.cargo` by default, which will be the default location that
+   is looked up for the `rustup` and `cargo` tools. If no `TOOLCHAIN_VERSION`
+   is specified (or if you are carrying out a cross-build), you need to
+   ensure that you added the target architecture to your default toolchain
+   via `rustup` if the architecture differs from the toolchain's architure.
+   If `TOOLCHAIN_VERSION` is specified and a cross-build is not carried out,
+   you will need to ensure the corresponding toolchain for the target
+   architecture is installed.
 *  `pkg-config` tool (or something that is compatible, set via
    `PKG_CONFIG` in the NMake command line if cross-compiling, or if in a
    non-standard location).  `PKG_CONFIG_PATH` in the NMake command line is