diff --git a/libcxx/docs/Status/Cxx20.rst b/libcxx/docs/Status/Cxx20.rst
--- a/libcxx/docs/Status/Cxx20.rst
+++ b/libcxx/docs/Status/Cxx20.rst
@@ -41,7 +41,9 @@
 .. note::
 
    .. [#note-P0600] P0600: The missing bits in P0600 are in |sect|\ [mem.res.class] and |sect|\ [mem.poly.allocator.class].
-   .. [#note-P0645] P0645: The paper is implemented but still marked as an incomplete feature. Not yet implemented LWG-issues will cause API and ABI breakage.
+   .. [#note-P0645] P0645: The paper is implemented but still marked as an incomplete feature
+      (the feature-test macro is not set and the libary is only available when built with ``LIBCXX_ENABLE_INCOMPLETE_FEATURES``).
+      Not yet implemented LWG-issues will cause API and ABI breakage.
    .. [#note-P0966] P0966: It was previously erroneously marked as complete in version 8.0. See `bug 45368 <https://llvm.org/PR45368>`__.
    .. [#note-P0619] P0619: Only sections D.8, D.9, D.10 and D.13 are implemented. Sections D.4, D.7, D.11, D.12, and D.14 remain undone.
    .. [#note-P0883] P0883: shared_ptr and floating-point changes weren't applied as they themselves aren't implemented yet.