Slightly clean up some lint infra code

* inline `LintBuffer::add_lint`, it only had a single use
* update a lint infra example code snippet
  * it used the wrong API (the snippet isn't tested)
  * presumably the arguments were updated from builder to diag struct style
    at some point without updating the method

compiler/rustc_macros/src/diagnostics/mod.rs

@@ -91,7 +91,7 @@ pub fn diagnostic_derive(mut s: Structure<'_>) -> TokenStream {
 /// Then, later, to emit the error:
 ///
 /// ```ignore (rust)
-/// cx.span_lint(INVALID_ATOMIC_ORDERING, fail_order_arg_span, AtomicOrderingInvalidLint {
+/// cx.emit_span_lint(INVALID_ATOMIC_ORDERING, fail_order_arg_span, AtomicOrderingInvalidLint {
 ///     method,
 ///     success_ordering,
 ///     fail_ordering,