Ask folks to use the Oxford comma in diagnostics

[onkoe]

Hey there! In Rust, it seems that most user-facing types of content prefer unambiguous grammar. However, there currently isn't any instruction to use the Oxford comma when writing lists of items within diagnostics.

I added that to avoid any ambiguity, such as that which I fixed in https://github.com/rust-lang/rust/pull/131038

Please let me know if any changes are necessary! :)