[concepts index] Describe exposition-only concepts

Fixes NB GB 136 (C++23 CD).

Author: tkoeppe

source/back.tex

@@ -119,7 +119,8 @@ \chapter{Bibliography}
 \clearpage
 \renewcommand{\preindexhook}{The bold page number for each entry is the page
 where the concept is defined.
-Other page numbers refer to pages where the concept is mentioned in the general text.\\}
+Other page numbers refer to pages where the concept is mentioned in the general text.
+Concepts whose name appears in \exposid{italics} are for exposition only.\\}
 {
 \raggedright
 \printindex[conceptindex]