docs: account for this context (#28720)

Issue number: N/A

---------

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## What is the current behavior?
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In https://github.com/ionic-team/ionic-framework/issues/28694 there was
some confusion around how to access `this` inside of a callback function
passed to a property on Ionic components. The root issue was due to how
the `this` context is determined with developers being responsible for
setting the appropriate `this` context.

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

- While this isn't an Ionic bug, I think it's worth calling out this
behavior so developers are aware of how to account for it.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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


Note: The link in the docs will not work until
https://github.com/ionic-team/ionic-docs/pull/3333 is merged.

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Co-authored-by: Amanda Johnston <90629384+amandaejohnston@users.noreply.github.com>

core/src/components/textarea/textarea.tsx

@@ -222,6 +222,9 @@ export class Textarea implements ComponentInterface {
   /**
    * A callback used to format the counter text.
    * By default the counter text is set to "itemLength / maxLength".
+   *
+   * See https://ionicframework.com/docs/troubleshooting/runtime#accessing-this
+   * if you need to access `this` from within the callback.
    */
   @Prop() counterFormatter?: (inputLength: number, maxLength: number) => string;