docs(forms): explain default validation messages (#88)

Author: rdlabo

projects/kit/docs/forms.md

@@ -14,7 +14,33 @@ import { KitIonicFormField } from '@rdlabo/ionic-angular-kit/forms';
 export class ProfilePage {}
 ```
 
-The adapter copies the first non-empty string validation message to Ionic's `errorText` property for `ion-input`, `ion-textarea`, `ion-select`, `ion-checkbox`, `ion-radio-group`, and `ion-toggle`. Validation wording and localization remain the application's responsibility. An explicit `errorText` or `[errorText]` binding takes precedence and prevents the adapter from being instantiated.
+The adapter copies the first non-empty explicit validation message to Ionic's `errorText` property for `ion-input`, `ion-textarea`, `ion-select`, `ion-checkbox`, `ion-radio-group`, and `ion-toggle`. When Angular's validator does not provide a message, the adapter derives a generic English message from the validation error `kind` and its constraint metadata. Built-in validators therefore need no message configuration:
+
+```ts
+readonly profileForm = form(this.profile, (path) => {
+  required(path.name);
+  email(path.email);
+  maxLength(path.name, 200);
+});
+```
+
+Unknown custom error kinds fall back to `Enter a valid value.`. Keep business-rule failures outside field validation. An explicit validation message still takes precedence for compatibility, and an explicit `errorText` or `[errorText]` binding prevents the adapter from being instantiated.
+
+Applications may replace the fallback resolver for localization without changing validator definitions:
+
+```ts
+import type { ValidationError } from '@angular/forms/signals';
+import { KIT_SIGNAL_FORM_ERROR_MESSAGE_RESOLVER } from '@rdlabo/ionic-angular-kit/forms';
+
+export const appConfig: ApplicationConfig = {
+  providers: [
+    {
+      provide: KIT_SIGNAL_FORM_ERROR_MESSAGE_RESOLVER,
+      useValue: (error: ValidationError) => localizedMessageFor(error),
+    },
+  ],
+};
+```
 
 Install the state-class configuration once at application bootstrap: