Tweak the wording on DOTNET_HOST_PATH docs to align with the new world (#49422)

* Tweak the wording on DOTNET_HOST_PATH docs to align with the new world

In .NET 10 / VS2026 we have a new invariant for DOTNET_HOST_PATH in builds that we should document

* Update docs/core/tools/dotnet-environment-variables.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update docs/core/tools/dotnet-environment-variables.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Fix whitespace

* Update docs/core/tools/dotnet-environment-variables.md

Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

* Update docs/core/tools/dotnet-environment-variables.md

Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

* Re-add NOTE

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

Author: 3 people

docs/core/tools/dotnet-environment-variables.md

@@ -183,15 +183,13 @@ These environment variables are used only when running apps via generated execut
 
 ### `DOTNET_HOST_PATH`
 
-Specifies the absolute path to a `dotnet` host (`dotnet.exe` on Windows, `dotnet` on Linux and macOS) that was used to launch the currently-running `dotnet` process. This is used by the .NET SDK to help tools that run during .NET SDK commands ensure they use the same `dotnet` runtime for any child `dotnet` processes they create for the duration of the command. Tools and MSBuild Tasks within the SDK that invoke binaries via the `dotnet` host are expected to honor this environment variable to ensure a consistent experience.
+Specifies the absolute path to a `dotnet` host (`dotnet.exe` on Windows, `dotnet` on Linux and macOS). This path represents either the host used to launch the currently running `dotnet` process, or the host that would be used when running `dotnet` commands for the currently building project when executing under MSBuild. The .NET SDK uses this variable to help tools that run during .NET SDK commands ensure they use the same `dotnet` host configuration for any child `dotnet` processes they create for the duration of the command. Tools and any MSBuild logic that run within a build and invoke binaries via the `dotnet` host are expected to honor this environment variable to ensure a consistent experience.
 
-Tools that invoke `dotnet` during an SDK command should use the following algorithm to locate it:
-
-- if `DOTNET_HOST_PATH` is set, use that value directly
-- otherwise, rely on `dotnet` via the system's `PATH`
+Starting in Visual Studio 2026, MSBuild in Visual Studio _also_ ensures that `DOTNET_HOST_PATH` is set for all builds of .NET SDK projects. For greatest consistency, all MSBuild tools and logic that want to use _the same dotnet binary_ as the one that spawned the build should rely on
+`DOTNET_HOST_PATH` and should consider emitting a diagnostic (warning or error) when the variable is not present.
 
 > [!NOTE]
-> `DOTNET_HOST_PATH` is not a general solution for locating the `dotnet` host. It is only intended to be used by tools that are invoked by the .NET SDK.
+> `DOTNET_HOST_PATH` is not a general solution for locating the `dotnet` host. It is only intended to be used by binaries and tools that are invoked by the .NET SDK or MSBuild.
 
 ### `DOTNET_LAUNCH_PROFILE`