Clarify validation errors and fix link for useDefaults.

docs/api.md

@@ -329,6 +329,8 @@ Options can have properties `separator` (string used to separate errors, ", " by
 
 In case of validation failure, Ajv assigns the array of errors to `errors` property of validation function (or to `errors` property of Ajv instance when `validate` or `validateSchema` methods were called). In case of [asynchronous validation](./guide/async-validation.md), the returned promise is rejected with exception `Ajv.ValidationError` that has `errors` property.
 
+Note, the `errors` property may not be set if validation fails due to a missing property, even if it is not required in the schema. In these cases, consider using [option `useDefaults`](./options.md#options).
+
 ### Error objects
 
 Each reported error is an object with the following properties:

docs/guide/modifying-data.md

@@ -154,7 +154,7 @@ See [discriminator](../json-schema.md#discriminator) keyword.
 
 ## Assigning defaults
 
-With [option `useDefaults`](./api.md#options) Ajv will assign values from `default` keyword in the schemas of `properties` and `items` (when it is the array of schemas) to the missing properties and items.
+With [option `useDefaults`](./options.md#options) Ajv will assign values from `default` keyword in the schemas of `properties` and `items` (when it is the array of schemas) to the missing properties and items.
 
 With the option value `"empty"` properties and items equal to `null` or `""` (empty string) will be considered missing and assigned defaults.