Improve validation errors docs

Author: damianmalczewski

docs/04-problem4j-spring/04-validation-errors.md

@@ -24,6 +24,8 @@ Library overrides default responses for `jakarta.validation` exceptions for both
 }
 ```
 
+## Adapted Constraint Violations
+
 More notably, for `@RequestParam`, `@RequestHeader` etc., there's a tweak that comes from settings configuration
 property `spring.validation.method.adapt-constraint-violations` to `true`. Enabling it, switches default validation to
 not rely on raw `ConstraintViolationException`, but rather on `MethodValidationException`, which contains more details
@@ -36,9 +38,10 @@ knowledge about internal technologies used by backend.
 ```java
 @Validated
 @RestController
-static class RequestParamController {
-  @GetMapping("/orders")
-  String endpoint(@RequestParam("customerId") @Size(min = 5, max = 30) String customerIdParam) {
+public class RequestParamController {
+
+  @GetMapping("/endpoint")      // note that query param in annotation different name from method param
+  public String endpoint(@Size(min = 5, max = 30) @RequestParam("customerId") String customerIdParam) {
     return "OK";
   }
 }
@@ -76,15 +79,14 @@ same goes for `@PathVariable`, `@RequestHeader`, `@CookieValue` etc.).
 [`MethodValidationProblemResolver`][MethodValidationProblemResolver] contains implementation of retrieving configured
 values from parameter annotations.
 
----
-
 > For Spring Boot versions lower than `3.5`, the above-mentioned property is not available and one must configure it
 > programmatically. Consider checking up `org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration`
 > and [Method Validation Exceptions][method-validation-exceptions] chapter of Spring Framework documentation.
 >
 > ```java
 > @Configuration
 > public class ExampleConfiguration {
+>
 >   @Bean
 >   public MethodValidationPostProcessor methodValidationPostProcessor() {
 >     MethodValidationPostProcessor processor = new MethodValidationPostProcessor();
@@ -96,11 +98,55 @@ values from parameter annotations.
 
 Method `setAdaptConstraintViolations` is available since Spring Framework `6.1` (therefore since Spring Boot `3.2`).
 
----
+## Supporting `@ModelAttribute` and `@BindParam`
 
 Similarly, for `@ModelAttribute` method arguments (object that groups query or form params), violation field names are
 resolved from `@BindParam` annotation if present on canonical constructor parameters.
 
+Let's see the following example.
+
+```java
+public class QueryBindObject {
+
+  @Size(min = 1, max = 5)
+  private final String text;
+
+  @Positive
+  private final Integer number;
+
+  public QueryBindObject(String text, @BindParam("num") Integer number) {
+    this.text = text;
+    this.number = number;
+  }
+
+  // ...
+}
+```
+
+Given `QueryBindObject`, declares, that a `num` query parameter will be resolved into `number` constructor argument.
+Therefore, for violation of that field name, we would like error to point to `num` field as being invalid instead of
+`number`.
+
+```java
+@Validated
+@RestController
+public class RequestParamController {
+
+  @GetMapping(path = "/endpoint")
+  public String endpoint(@ModelAttribute @Valid QueryBindObject query) {
+    return "OK";
+  }
+}
+```
+
+While resolving constraint violation error, Problem4J will resolve `@BindParam` annotations present on canonical
+constructor of `@ModelAttribute`-annotated request argument. Node that if there's no annotation on controller method,
+Spring automatically assumes it to be a `@ModelAttribute`.
+
+**Note** that given object must have exactly one, full-args constructor if it is an ordinary `class`. If multiple
+constructors are present, Problem4J will skip evaluation of `@BindParam`. If it is a `record`, then actual arguments
+will be taken from **record's canonical constructor**, and additional constructors are ignored.
+
 [MethodValidationProblemResolver]: https://github.com/problem4j/problem4j-spring/blob/v2.1.0/problem4j-spring-web/src/main/java/io/github/problem4j/spring/web/resolver/MethodValidationProblemResolver.java
 
 [method-validation-exceptions]: https://docs.spring.io/spring-framework/reference/core/validation/beanvalidation.html#validation-beanvalidation-spring-method-exceptions