Doc-only version of #58850 (runtime deprecation).

The DEP0137 is going to be End-of-Life in the next major release.

I think we should align closing fs.Dir on GC and deprecate it as well. Now that explicit resource management is available, closing dirs properly should become easier for userland.

Note that every time a Dir is closed on GC, it already emits a warning. Alternatively we can make this deprecation runtime right away, I don't have strong opinion on this.