Document RFC 3681: Default field values

[agraven]

The feature is missing documentation

Link to tracking issue: rust-lang/rust#132162

(I intend to implement this and am filing this issue for tracking purposes. I do not seem able to assign it to myself.)

[ehuss]

Thanks for checking in! I believe there is already a PR for this at #1766.

Also, we typically don't have tracking issues in this repo since the rust-lang/rust tracking issues normally should include checkbox for it (I updated it to link).

[agraven]

I see, I unfortunately managed to miss that when searching for existing issues/PRs. I should probably just close this issue then, right?

[ehuss]

Yea, let's close. Thanks!