Add docs.

.chronus/changes/AvoidAdditionalProperties-2024-1-23-10-13-55.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-azure-resource-manager"
+---
+
+Add `arm-avoid-additional-properties` rule.

docs/libraries/azure-resource-manager/rules/avoid-additional-properties.md

@@ -0,0 +1,27 @@
+---
+title: avoid-additional-properties
+---
+
+```text title=- Full name-
+@azure-tools/typespec-azure-resource-manager/avoid-additional-properties
+```
+
+ARM requires Resource provider teams to define types explicitly. This is to ensure good customer experience in terms of the discoverability of concrete type definitions.
+
+#### ❌ Incorrect
+
+```tsp
+model Address extends Record<string> {}
+```
+
+#### ✅ Correct
+
+```tsp
+model Address {
+  street: string;
+  city: string;
+  state: string;
+  country: string;
+  postalCode: string;
+}
+```

packages/typespec-azure-resource-manager/src/rules/arm-avoid-additional-properties.ts

@@ -8,8 +8,13 @@ export const armAvoidAdditionalPropertiesRule = createRule({
   name: "arm-avoid-additional-properties",
   severity: "warning",
   description: "Avoid use of additional properties in ARM resources.",
+  url: "https://azure.github.io/typespec-azure/docs/libraries/azure-resource-manager/rules/avoid-additional-properties",
   messages: {
-    default: "...",
+    default:
+      "Definitions must not be of type Record. ARM requires Resource provider teams to define types explicitly.",
+    extends:
+      "Definitions must not extend type Record. ARM requires Resource provider teams to define types explicitly.",
+    is: "Definitions must not equate to type Record. ARM requires Resource provider teams to define types explicitly.",
   },
   create(context): SemanticNodeListener {
     return {
@@ -26,12 +31,14 @@ export const armAvoidAdditionalPropertiesRule = createRule({
           context.reportDiagnostic({
             code: "arm-avoid-additional-properties",
             target: model,
+            messageId: "extends",
           });
         }
         if (model.sourceModel !== undefined && isRecordType(model.sourceModel)) {
           context.reportDiagnostic({
             code: "arm-avoid-additional-properties",
             target: model,
+            messageId: "is",
           });
         }
       },