docs: Clarify forbidUnknownValues option

README.md

@@ -139,7 +139,7 @@ export interface ValidatorOptions {
 }
 ```
 
-> It's highly advised to set `forbidUnknownValues: true` as it will prevent unknown objects from passing validation.
+> It's highly advised to set `forbidUnknownValues: true` as it will prevent plain javascript objects (eg. `{}`) from passing validation.
 
 ## Validation errors
 

src/validation/ValidatorOptions.ts

@@ -72,7 +72,12 @@ export interface ValidatorOptions {
   };
 
   /**
-   * Settings true will cause fail validation of unknown objects.
+   * Fails validation for objects unknown to class-validator. Defaults to false.
+   * 
+   * For instance, since a plain empty object has no annotations used for validation:
+   * - `validate({})` // passes
+   * - `validate({}, { forbidUnknownValues: true })` // fails.
+   * - `validate(new SomeAnnotatedEmptyClass(), { forbidUnknownValues: true })` // passes.
    */
   forbidUnknownValues?: boolean;