Add release notes entry for FS3879 warning

Co-authored-by: abonie <[email protected]>

Author: Copilot

docs/release-notes/.FSharp.Compiler.Service/11.0.0.md

@@ -20,6 +20,7 @@
 * Add FSharpCodeCompletionOptions ([PR #19030](https://github.com/dotnet/fsharp/pull/19030))
 * Type checker: recover on checking binding parameter constraints ([#19046](https://github.com/dotnet/fsharp/pull/19046))
 * Debugger: provide breakpoint ranges for short lambdas ([#19067](https://github.com/dotnet/fsharp/pull/19067))
+* Add warning FS3879 for XML documentation comments not positioned as first non-whitespace on line. ([PR #18891](https://github.com/dotnet/fsharp/pull/18891))
 
 ### Changed