Address review comments

- Add doc comment to new type
- Restore "only supported directly in conditions of `if` and `while` expressions" note
- Rename variant with clearer name

compiler/rustc_parse/src/errors.rs

@@ -392,6 +392,7 @@ pub(crate) struct IfExpressionMissingCondition {
 
 #[derive(Diagnostic)]
 #[diag(parse_expected_expression_found_let)]
+#[note]
 pub(crate) struct ExpectedExpressionFoundLet {
     #[primary_span]
     pub span: Span,