Clarify whether router.pathname includes basePath (#24675)

The docs were vague on this -- `router.asPath` mentions that it doesn't include `basePath` but `router.pathname` didn't say either way.

## Documentation / Examples

- [ ] Make sure the linting passes: I haven't checked; I presume you have a CI bot for this.

docs/api-reference/next/router.md

@@ -41,7 +41,7 @@ export default ActiveLink
 
 The following is the definition of the `router` object returned by both [`useRouter`](#useRouter) and [`withRouter`](#withRouter):
 
-- `pathname`: `String` - Current route. That is the path of the page in `/pages`
+- `pathname`: `String` - Current route. That is the path of the page in `/pages`, the configured `basePath` or `locale` is not included.
 - `query`: `Object` - The query string parsed to an object. It will be an empty object during prerendering if the page doesn't have [data fetching requirements](/docs/basic-features/data-fetching.md). Defaults to `{}`
 - `asPath`: `String` - The path (including the query) shown in the browser without the configured `basePath` or `locale`.
 - `isFallback`: `boolean` - Whether the current page is in [fallback mode](/docs/basic-features/data-fetching.md#fallback-pages).