Tighten up ErrorGuaranteed handling.

- In `emit_producing_error_guaranteed`, only allow `Level::Error`.
- In `emit_diagnostic`, only produce `ErrorGuaranteed` for `Level` and
  `DelayedBug`. (Not `Bug` or `Fatal`. They don't need it, because the
  relevant `emit` methods abort.)
- Add/update various comments.

compiler/rustc_span/src/lib.rs

@@ -2477,9 +2477,8 @@ where
 pub struct ErrorGuaranteed(());
 
 impl ErrorGuaranteed {
-    /// To be used only if you really know what you are doing... ideally, we would find a way to
-    /// eliminate all calls to this method.
-    #[deprecated = "`Session::span_delayed_bug` should be preferred over this function"]
+    /// Don't use this outside of `DiagCtxtInner::emit_diagnostic`!
+    #[deprecated = "should only be used in `DiagCtxtInner::emit_diagnostic`"]
     pub fn unchecked_claim_error_was_emitted() -> Self {
         ErrorGuaranteed(())
     }