Add troubleshoot guide for enum -> union migration (#506)

Co-authored-by: Mark Cowlishaw <markcowl@microsoft.com>

docs/troubleshoot/enum-not-extensible.md

@@ -0,0 +1,32 @@
+# My enums are not extensible anymore
+
+## Symptoms
+
+I had an enum that used to generate `x-ms-enum.modelAsString: true` but now it is generating as `x-ms-enum.modelAsString: false` and I see a warning message `@azure-tools/typespec-azure-core/no-enum`
+
+## Cause
+
+Azure stopped treating enums as extensible.
+
+## Workaround
+
+To define an extensible enum you will need instead to use a `union` where one of the variants is `string`.
+If you see the linter warning [`@azure-tools/typespec-azure-core/no-enum`](https://tspwebsitepr.z22.web.core.windows.net/typespec-azure/prs/389/docs/next/libraries/azure-core/rules/no-enum) it also offers an automatic codefix (click the (ℹ) bulb in VS Code)
+For example
+
+```tsp
+enum PetKind {
+  Cat,
+  Dog,
+}
+```
+
+should be converted to
+
+```tsp
+union PetKind {
+  Cat: "Cat",
+  Dog: "Dog",
+  string,
+}
+```