iter 22-tidy.6.1 fixup: cover abstract-method + instance-override branches; doc + fallback comments

examples/test_22b2_instance_present.prose.txt

@@ -0,0 +1,17 @@
+// module test_22b2_instance_present
+
+class Show a {
+  fn show(x: a) -> Str
+}
+
+instance Show Int {
+  fn show {
+    |x: a| -> Str {
+      "n"
+    }
+  }
+}
+
+fn main() -> Str {
+  show(42)
+}