diff --git a/libcxx/docs/index.rst b/libcxx/docs/index.rst
--- a/libcxx/docs/index.rst
+++ b/libcxx/docs/index.rst
@@ -118,6 +118,11 @@
 Windows         x86_64
 =============== ========================= ============================
 
+These are platform/architecture combinations that are continuously tested, that
+are formally supported. In practice, the library also is regularly used on
+other architectures and platforms, but as they're not continuously tested
+they're not listed as formally supported.
+
 Generally speaking, libc++ should work on any platform that provides a fairly complete
 C Standard Library. It is also possible to turn off parts of the library for use on
 systems that provide incomplete support.