Rollup merge of #122675 - tmfink:doc-clarify, r=scottmcm

core: document default attribute stabilization

As of now, the first release which stabilized the `#[default]` macro for the deriving the `Default` trait for enus is not documented.
I have had to search the [`RELEASES.md`](https://github.com/rust-lang/rust/blob/master/RELEASES.md) when making sure my code would be accepted by an older Rust compiler.

I just added a line in the doc comment since, as far as I know, there's no option to pass to the `#[stable()]` attribute.

I am open to improvements in the wording.

Author: matthiaskrgr