Explain how to migrate sln to slnx

Author: AArnott

.github/prompts/template-release-notes.md

@@ -8,5 +8,13 @@ This file is referenced by update-library-template.prompt.md and should remain i
 Never leave a Library.slnx file in the repository.
 You might even see one there even though this particular merge didn't bring it in.
 This can be an artifact of having renamed Library.sln to Library.slnx in the template repo, but ultimately the receiving repo should have only one .sln or .slnx file, with a better name than `Library`.
-If you can confidently do so, go ahead and migrate their `.sln` to an `.slnx` with their original name.
-If you can't, just delete `Library.slnx` anyway.
+Delete any `Library.slnx` that you see.
+Migrate an `.sln` in the repo root to `.slnx` using this command:
+
+```ps1
+dotnet solution EXISTING.sln migrate
+```
+
+This will create an EXISTING.slnx file. `git add` that file, then `git rm` the old `.sln` file.
+Sometimes a repo will reference the sln filename in a script or doc somewhere.
+Search the repo for such references and update them to the slnx file.