Update router.prefetch documentation to include locale option

[jonohewitt]

Includes some basic documentation for setting the locale as an option within router.prefetch. I was quite confused when I was looking to use this feature recently and couldn't see it in the docs.

Wording is adapted from:

next.js/docs/api-reference/next/link.md

Line 64 in a6bc387

- `locale` - The active locale is automatically prepended. `locale` allows for providing a different locale. When `false` `href` has to include the locale as the default behavior is disabled.

There is also an undocumented priority boolean option described in the typescript types however having just tested it, it doesn't seem to have any effect so perhaps it's not ready for documentation and I have left it out.

Feature

• Documentation added

[ijjk]

Thanks!