docs(input, searchbar, textarea): improve docs for managing focus (#28614)

Issue number: Related to #18132 

---------

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## What is the current behavior?
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The documentation about the `autofocus` prop is unclear and does not
accurately reflect how it actually works across browsers and devices.

## What is the new behavior?
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this PR. -->

- The documentation for `autofocus` and `setFocus` are more detailed.
- The documentation links to the relevant page in the docs.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->

core/src/components/textarea/textarea.tsx

@@ -93,7 +93,9 @@ export class Textarea implements ComponentInterface {
   @Prop() autocapitalize = 'none';
 
   /**
-   * This Boolean attribute lets you specify that a form control should have input focus when the page loads.
+   * Sets the [`autofocus` attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/autofocus) on the native input element.
+   *
+   * This may not be sufficient for the element to be focused on page load. See [managing focus](/docs/developing/managing-focus) for more information.
    */
   @Prop() autofocus = false;
 
@@ -372,6 +374,8 @@ export class Textarea implements ComponentInterface {
   /**
    * Sets focus on the native `textarea` in `ion-textarea`. Use this method instead of the global
    * `textarea.focus()`.
+   *
+   * See [managing focus](/docs/developing/managing-focus) for more information.
    */
   @Method()
   async setFocus() {