Tweak `x-ms-client-default` docs (#4334)

common/changes/@autorest/core/docs-x-ms-client-default_2021-10-08-18-06.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "packageName": "@autorest/core",
+      "comment": "**Fix** --help crash",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@autorest/core",
+  "email": "tiguerin@microsoft.com"
+}

docs/extensions/readme.md

@@ -850,10 +850,13 @@ Note: The extension is not tight to this particular scenario (you could model an
 
 ## x-ms-client-default
 
-Set the default value for a property or a parameter.
+Set the default value for a property or a parameter on the client.
+With this extension, you can set a default value for a property or parameter that is independent of how the property / parameter's schema is handling a default.
 
-With this extension, you can set a default value for a property or parameter that is independent of how the property / parameter's schema is handling a default. This is different than the `default` value
-you can specify
+**`x-ms-client-default` vs `default`:**
+
+- `default`: This represent the server default. From the point of view of the client this is just documentation, it will be ignored.
+- `x-ms-client-default`: This represent the default on the client. It can be used to make a required property optional.
 
 **Parent element**: [Parameter Objects](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#parameterObject) or [Property on the Schema Definition](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#schemaObject).
 

packages/extensions/core/src/lib/autorest-core.ts

@@ -152,7 +152,9 @@ export class AutoRest extends EventEmitter {
             // if this is using perform-load we don't need to require files.
             // if it's using batch, we might not have files in the main body
             if (view.config.raw["perform-load"] !== false) {
-              return new Exception("No input files provided.\n\nUse --help to get help information.");
+              return new Exception(
+                "No input files provided.\n\nUse --help to get help information or see https://aka.ms/autorest/cli for additional documentation",
+              );
             }
           }
         }

packages/extensions/core/src/lib/plugins/help.ts

@@ -9,13 +9,15 @@ import { PipelinePlugin } from "../pipeline/common";
 /* @internal */
 export function createHelpPlugin(): PipelinePlugin {
   return async (config) => {
-    const help: { [helpKey: string]: any } = config.GetEntry(<any>"help-content");
-    for (const helpKey of Object.keys(help).sort()) {
-      config.GeneratedFile.Dispatch({
-        type: "help",
-        uri: `${helpKey}.json`,
-        content: JSON.stringify(help[helpKey]),
-      });
+    const help: { [helpKey: string]: any } = config.GetEntry("help-content");
+    if (help) {
+      for (const helpKey of Object.keys(help).sort()) {
+        config.GeneratedFile.Dispatch({
+          type: "help",
+          uri: `${helpKey}.json`,
+          content: JSON.stringify(help[helpKey]),
+        });
+      }
     }
     return new QuickDataSource([]);
   };