Add dotnet.config to known RC1 issues (#10073)

* Add dotnet.config to known RC1 issues

* Update release-notes/10.0/known-issues.md

Co-authored-by: Amaury Levé <amauryleve@microsoft.com>

* Update known-issues.md

* Expand example

* Update P3 notes as well

* Fix

---------

Co-authored-by: Amaury Levé <amauryleve@microsoft.com>

release-notes/10.0/known-issues.md

@@ -26,3 +26,37 @@ Add the following to a `Directory.Build.props` file that is imported in all MAUI
 ```
 
 The behavior will be **fixed in the .NET 10 preview 5** release. The workaround should be removed after that release.
+
+## `dotnet.config` will be removed in favor of `global.json`
+
+In .NET 10, we introduced `dotnet.config` file that is used to set the test runner and is the way to opt-in using `dotnet test` for Microsoft.Testing.Platform. This is already shipping in RC1, but will be removed in RC2.
+
+The `dotnet.config` used to look like:
+
+```ini
+[dotnet.test.runner]
+name = "Microsoft.Testing.Platform"
+```
+
+With `global.json`, this becomes:
+
+```json
+{
+  "test": {
+    "runner": "Microsoft.Testing.Platform"
+  }
+}
+```
+
+A `global.json` that sets SDK version along with the test runner will look like:
+
+```json
+{
+  "sdk": {
+    "version": "<version>"
+  },
+  "test": {
+    "runner": "Microsoft.Testing.Platform"
+  }
+}
+```

release-notes/10.0/preview/preview3/sdk.md

@@ -90,6 +90,9 @@ The container images the .NET SDK produces today are either in a Docker-specific
 
 ### Support for Microsoft Testing Platform in `dotnet test`
 
+> [!IMPORTANT]
+> In RC2, we are moving from `dotnet.config` to `global.json`, and we are removing `dotnet.config` support completely.
+
 Starting with .NET 10 Preview 3, `dotnet test` now natively supports the [Microsoft.Testing.Platform](https://aka.ms/mtp-overview). You can enable this feature by adding the following configuration to your dotnet.config file (which itself is a [draft specification](https://github.com/dotnet/designs/pull/328) that we would love your feedback on):
 
 ```ini