Add doc about ref-struct lifetime variance

[agocke]

This should wrap up any formalization holes we have around lifetime variance and nested ref fields

[agocke]

@jaredpar

[jaredpar]

Kinda wary about the number of lifetime docs we have now. Debating if we should just add this directly into the low level struct improvements doc or put it in the expand ref one.

[agocke]

Seems like a fine idea to me. Happy to centralize. I mention at the top that it’s based on the low-level improvements doc.

[jaredpar]

As long as it's describing existing behavior we should just update the existing doc.