docs: clarify meaning of injectable decorator (#31573)

PR Close #31573

aio/content/guide/cheatsheet.md

@@ -185,8 +185,7 @@ is available to <code>declarations</code> of this module.</p>
 </td>
 </tr><tr>
 <td><code><b>@Injectable()</b><br>class MyService() {}</code></td>
-<td><p>Declares that a class has dependencies that should be injected into the constructor when the dependency injector is creating an instance of this class.
-</p>
+<td><p>Declares that a class can be provided and injected by other classes. Without this decorator, the compiler won't generate enough metadata to allow the class to be created properly when it's injected somewhere.</p>
 </td>
 </tr>
 </tbody></table>

packages/core/src/di/injectable.ts

@@ -31,15 +31,20 @@ export type InjectableProvider = ValueSansProvider | ExistingSansProvider |
  */
 export interface InjectableDecorator {
   /**
-   * Decorator that marks a class as available to `Injector` for creation.
+   * Decorator that marks a class as available to be
+   * provided and injected as a dependency.
    *
    * @see [Introduction to Services and DI](guide/architecture-services)
    * @see [Dependency Injection Guide](guide/dependency-injection)
    *
    * @usageNotes
    *
-   * The following example shows how service classes are properly marked as
-   * injectable.
+   * Marking a class with `@Injectable` ensures that the compiler
+   * will generate the necessary metadata to create the class's
+   * dependencies when the class is injected.
+   *
+   * The following example shows how a service class is properly
+   *  marked so that a supporting service can be injected upon creation.
    *
    * <code-example path="core/di/ts/metadata_spec.ts" region="Injectable"
    *  linenums="false"></code-example>