Add note about using the /_error page directly to custom error page a…

…rticle (#39671)

See #39144

It's not completely clear that `/_error` should not be used as a normal page. Added a note under `Caveats` in the custom error page article

docs/advanced-features/custom-error-page.md

@@ -98,3 +98,4 @@ If you have a custom `Error` component be sure to import that one instead. `next
 ### Caveats
 
 - `Error` does not currently support Next.js [Data Fetching methods](/docs/basic-features/data-fetching.md) like [`getStaticProps`](/docs/basic-features/data-fetching/get-static-props.md) or [`getServerSideProps`](/docs/basic-features/data-fetching/get-server-side-props.md).
+- `_error`, like `_app`, is a reserved pathname. `_error` is used to define the customized layouts and behaviors of the error pages. `/_error` will render 404 when accessed directly via [routing](/docs/routing/introduction) or rendering in a [custom server](/docs/advanced-features/custom-server).