docs: add example for disableCorePrefetching (#9852)

<!--
  Q. Which branch should I use for my pull request?
  A. Use `master` branch (probably).

  Q. Which branch if my change is a bug fix for Gatsby v1?
  A. In this case, you should use the `v1` branch

  Q. Which branch if I'm still not sure?
  A. Use `master` branch. Ask in the PR if you're not sure and a Gatsby maintainer will be happy to help :)

  Note: We will only accept bug fixes for Gatsby v1. New features should be added to Gatsby v2.

  Learn more about contributing: https://www.gatsbyjs.org/docs/how-to-contribute/
-->
This closes #9827

packages/gatsby/src/utils/api-browser-docs.js

@@ -176,6 +176,8 @@ exports.onPostPrefetchPathname = true
 /**
  * Plugins can take over prefetching logic. If they do, they should call this
  * to disable the now duplicate core prefetching logic.
+ * @example
+ * exports.disableCorePrefetching = () => true
  */
 exports.disableCorePrefetching = true