docs(blocks): teach a validation shape the input node actually declares (#5229)

content/docs/blocks/forms.mdx

@@ -39,19 +39,58 @@ Customize form blocks for your application:
 
 ### Add Validation
 
+The blocks on this page are built from plain `input` nodes, and an `input`
+declares its own validation keys — `required`, `pattern`, `maxLength`, `min`,
+`max` and `step` — which the renderer forwards to the native HTML input
+attributes:
+
 ```json
 {
  "type": "input",
  "name": "email",
  "inputType": "email",
  "required": true,
- "validation": {
- "pattern": "^[a-z0-9._%+-]+@[a-z0-9.-]+\\.[a-z]{2,}$",
- "message": "Please enter a valid email address"
- }
+ "pattern": "^[a-z0-9._%+-]+@[a-z0-9.-]+\\.[a-z]{2,}$"
+}
+```
+
+`inputType: "email"` already gets the browser's own email check; `pattern`
+tightens it. The browser enforces these constraints when the input is submitted
+inside a `<form>`.
+
+An `input` node has **no `validation` key**. Rule objects with custom messages
+belong to the [form component](/docs/components/form/form), whose `fields[]`
+entries carry them. Every rule there is a `{ value, message }` object, and
+`validation.required` supplies the message only — whether the field is required
+is decided by `required` on the field itself:
+
+```json
+{
+ "type": "form",
+ "fields": [
+ {
+ "name": "message",
+ "type": "textarea",
+ "label": "How can we help?",
+ "required": true,
+ "validation": {
+ "required": "Please tell us how we can help",
+ "minLength": { "value": 20, "message": "Please use at least 20 characters" },
+ "maxLength": { "value": 2000, "message": "Please keep it under 2000 characters" }
+ }
+ }
+ ]
 }
 ```
 
+`pattern` is the one rule that route cannot take from JSON: react-hook-form runs
+it only when the rule's `value` is a compiled `RegExp`, and no JSON document can
+hold one. Declare the pattern on the object field's metadata instead (`pattern`,
+a string, which `@object-ui/fields` compiles before the rule reaches the form),
+or in a TypeScript-authored schema pass the real thing —
+`pattern: { value: /.../, message: '...' }`. The full rule table is in the
+[Form Plugin](/docs/plugins/plugin-form) reference.
+
 ### Add Submit Action
 
 ```json