Add docs for field default values

[bluebear94]

Tracking issue: rust-lang/rust#132162

[traviscross]

cc @estebank

[traviscross]

@bluebear94: You've been contributing a number of things recently. Thanks for that. We're on the spec call at the moment, and would be interested in talking more with you. If you're able to join us on Zulip, that'd be great. You can then reach out to #t-spec to introduce yourself to the group or to TC directly.

[ehuss]

@estebank In particular, I'm curious what the status of this feature is in terms of how close it is to stabilization? Is this something you can help with review? I'm generally shy from doing reviews if there are still changes forthcoming, and I have no context on this feature.

[compiler-errors]

From a compiler perspective, it's probably still worth baking the feature for some time, since it's still pretty new implementation-wise. But I don't think it's going to change much, so it's probably fine to keep this open and tagged as waiting for stabilization and revisit it for tweaks when the stabilization happens.