Note in HOWITWORKS about `opaque-placeholder`.

syndicate/HOWITWORKS.md

@@ -543,6 +543,10 @@ interested in the `present` constructor.
       trigger its "matching" inner endpoint, but not the inner
       endpoint of the other endpoint, even though you might expect the
       relayed assertion to do so.
+    - There's also a need for `(opaque-placeholder)`s to frustrate
+      constant-matching against literal `(discard)` in cases of
+      visibility restriction. See commit b4f1d36 and test case
+      `test/core/nesting-confusion-2.rkt`.
 
  - there's more to say about the implementation of the *dataspace*
    itself, not just the index structures. For example, the care that