docs: add global form state signals to signal forms guide

Add a new section for 'Form State Signals' in the signal forms essential guide. This documents global signals such as dirty(), valid(), invalid(), pending(), and touched() at the form level, as requested in issue angular#69544.

Fixes angular#69544

Author: aparzi

adev/src/content/introduction/essentials/signal-forms.md

@@ -22,16 +22,17 @@ const loginModel = signal<LoginData>({
 });
 ```
 
-### 2. Pass the form model to `form()` to create a `FieldTree`
+### 2. Create a `FieldTree` with `form()`
 
-Then, you pass your form model into the `form()` function to create a **field tree** - an object structure that mirrors your model's shape, allowing you to access fields with dot notation:
+Then, you pass your form model into the `form()` function to create a **field tree** - an object structure that mirrors your model's shape, allowing you to access fields with dot notation.
+
+Both the root form object and its nested properties are `FieldTree` nodes:
 
 ```ts
 const loginForm = form(loginModel);
 
-// Access fields directly by property name
-loginForm.email;
-loginForm.password;
+// loginForm is a FieldTree
+// loginForm.email is also a FieldTree
 ```
 
 ### 3. Bind HTML inputs with `[formField]` directive
@@ -47,18 +48,20 @@ As a result, user changes (such as typing in the field) automatically updates th
 
 NOTE: The `[formField]` directive also syncs field state for attributes like `required`, `disabled`, and `readonly` when appropriate.
 
-### 4. Read field values with `value()`
+### 4. Read state with `FieldTree` signals
 
-You can access field state by calling the field as a function. This returns a `FieldState` object containing reactive signals for the field's value, validation status, and interaction state:
+You can access state for any part of the tree by calling the `FieldTree` node as a function. This returns a state object containing reactive signals for the value, validation status, and interaction state:
 
 ```ts
-loginForm.email(); // Returns FieldState with value(), valid(), touched(), etc.
+loginForm(); // Returns state for the whole form
+loginForm.email(); // Returns state for the email field
 ```
 
-To read the field's current value, access the `value()` signal:
+To read the current value, access the `value()` signal:
 
 ```html
-<!-- Render form value that updates automatically as user types -->
+<!-- Render values that update automatically as user types -->
+<p>Form value: {{ loginForm().value() | json }}</p>
 <p>Email: {{ loginForm.email().value() }}</p>
 ```
 
@@ -67,9 +70,9 @@ To read the field's current value, access the `value()` signal:
 const currentEmail = loginForm.email().value();
 ```
 
-### 5. Update field values with `set()`
+### 5. Update values with `set()`
 
-You can programmatically update a field's value using the `value.set()` method. This updates both the field and the underlying model signal:
+You can programmatically update values using the `value.set()` method on any node. This updates both the `FieldTree` and the underlying model signal:
 
 ```ts
 // Update the value programmatically
@@ -235,7 +238,22 @@ required(schemaPath.email, {message: 'Email is required'});
 email(schemaPath.email, {message: 'Please enter a valid email address'});
 ```
 
-Each form field exposes its validation state through signals. For example, you can check `field().valid()` to see if validation passes, `field().touched()` to see if the user has interacted with it, and `field().errors()` to get the list of validation errors.
+Each node in the `FieldTree` exposes its validation and interaction state through reactive signals.
+
+### FieldTree State Signals
+
+Every node in the tree, including the root form object, provides the same signals to track its state. Since every node is a `FieldTree`, the API for monitoring validity and interaction is identical at every level.
+
+| State        | Description                                                                     |
+| ------------ | ------------------------------------------------------------------------------- |
+| `valid()`    | Returns `true` if the node passes all validation rules                          |
+| `invalid()`  | Returns `true` if there are validation errors                                   |
+| `pending()`  | Returns `true` if async validation is in progress                               |
+| `touched()`  | Returns `true` if the user has focused and blurred the field or any child field |
+| `dirty()`    | Returns `true` if the value has been changed by the user                        |
+| `disabled()` | Returns `true` if the node is disabled                                          |
+| `readonly()` | Returns `true` if the node is readonly                                          |
+| `errors()`   | Returns an array of validation errors with `kind` and `message` properties      |
 
 Here's a complete example:
 
@@ -245,20 +263,6 @@ Here's a complete example:
   <docs-code header="app.css" path="adev/src/content/examples/signal-forms/src/login-validation/app/app.css"/>
 </docs-code-multifile>
 
-### Field State Signals
-
-Every `field()` provides these state signals:
-
-| State        | Description                                                                |
-| ------------ | -------------------------------------------------------------------------- |
-| `valid()`    | Returns `true` if the field passes all validation rules                    |
-| `touched()`  | Returns `true` if the user has focused and blurred the field               |
-| `dirty()`    | Returns `true` if the user has changed the value                           |
-| `disabled()` | Returns `true` if the field is disabled                                    |
-| `readonly()` | Returns `true` if the field is readonly                                    |
-| `pending()`  | Returns `true` if async validation is in progress                          |
-| `errors()`   | Returns an array of validation errors with `kind` and `message` properties |
-
 ## Next steps
 
 To learn more about Signal Forms and how it works, check out the in-depth guides: