docs: Adds signal type checking documentation

Author: SkyZeroZx

adev/src/content/guide/signals/overview.md

@@ -195,6 +195,29 @@ Equality functions can be provided to both writable and computed signals.
 
 HELPFUL: By default, signals use referential equality ([`Object.is()`](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object/is) comparison).
 
+### Type checking signals
+
+You can use `isSignal` to check if a value is a `Signal`:
+
+```ts
+const count = signal(0);
+const doubled = computed(() => count() * 2);
+
+isSignal(count); // true
+isSignal(doubled); // true
+isSignal(42); // false
+```
+
+To specifically check if a signal is writable, use `isWritableSignal`:
+
+```ts
+const count = signal(0);
+const doubled = computed(() => count() * 2);
+
+isWritableSignal(count); // true
+isWritableSignal(doubled); // false
+```
+
 ### Reading without tracking dependencies
 
 Rarely, you may want to execute code which may read signals within a reactive function such as `computed` or `effect` _without_ creating a dependency.

packages/core/src/render3/reactivity/api.ts

@@ -17,6 +17,8 @@ import type {WritableSignal} from './signal';
  *
  * Ordinary values can be turned into `Signal`s with the `signal` function.
  *
+ * @see [What are signals?](guide/signals#what-are-signals)
+ *
  * @publicApi 17.0
  */
 export type Signal<T> = (() => T) & {
@@ -26,6 +28,8 @@ export type Signal<T> = (() => T) & {
 /**
  * Checks if the given `value` is a reactive `Signal`.
  *
+ * @see [Type checking signals](guide/signals#type-checking-signals)
+ *
  * @publicApi 17.0
  */
 export function isSignal(value: unknown): value is Signal<unknown> {
@@ -35,13 +39,17 @@ export function isSignal(value: unknown): value is Signal<unknown> {
 /**
  * A comparison function which can determine if two values are equal.
  *
+ * @see [Signal equality functions](guide/signals#signal-equality-functions)
+ *
  * @publicApi 17.0
  */
 export type ValueEqualityFn<T> = (a: T, b: T) => boolean;
 
 /**
  * Checks if the given `value` is a writeable signal.
  *
+ * @see [Type checking signals](guide/signals#type-checking-signals)
+ *
  * @publicApi 21.1
  */
 export function isWritableSignal(value: unknown): value is WritableSignal<unknown> {