docs(aio): add short description for entryComponents (angular#21360)

PR Close angular#21360

Author: VadimDez

aio/content/guide/cheatsheet.md

@@ -44,6 +44,9 @@ is available to <code>declarations</code> of this module.</p>
 <td><p>List of dependency injection providers visible both to the contents of this module and to importers of this module.</p>
 </td>
 </tr><tr>
+<td><code><b>entryComponents:</b> [SomeComponent, OtherComponent]</code></td>
+<td><p>List of components not referenced in any reachable template, for example dynamically created from code.</p></td>
+</tr><tr>
 <td><code><b>bootstrap:</b> [MyAppComponent]</code></td>
 <td><p>List of components to bootstrap when this module is bootstrapped.</p>
 </td>