diagnostics: lists should use the Oxford comma

we encountered this in https://github.com/rust-lang/rust/pull/131038

kinda felt like an unspoken rule so here's a more concrete
clarification.

src/diagnostics.md

@@ -156,6 +156,8 @@ use an error-level lint instead of a fixed error.
   compiler messages are an important learning tool.
 - When talking about the compiler, call it `the compiler`, not `Rust` or
   `rustc`.
+- Use the [Oxford comma](https://en.wikipedia.org/wiki/Serial_comma) when
+  writing lists of items.
 
 ### Lint naming