diff -u b/core/lib/Drupal/Core/Extension/ModuleUninstallValidatorInterface.php b/core/lib/Drupal/Core/Extension/ModuleUninstallValidatorInterface.php
--- b/core/lib/Drupal/Core/Extension/ModuleUninstallValidatorInterface.php
+++ b/core/lib/Drupal/Core/Extension/ModuleUninstallValidatorInterface.php
@@ -9,14 +9,19 @@
 
 /**
  * Common interface for module uninstall validators.
+ *
+ * A module uninstall validator must implement this interface and be defined in
+ * a Drupal service (see the @link container Services and container
+ * topic @endlink) that is tagged module_install.uninstall_validator.
  */
 interface ModuleUninstallValidatorInterface {
 
   /**
    * Determines the reasons a module can not be uninstalled.
    *
-   * Use this method to return a list of reasons why a module cannot be
-   * uninstalled in the current state of the system.
+   * This method must either return a list of reasons why a module cannot be
+   * uninstalled in the current state of the system, or return an empty list,
+   * which means that it is safe to uninstall the module.
    *
    * Example usage:
    * @code