Can I add a footnote (or a paragraph of text) to an index?

Writing about a phenomenon X, I used X acronim too many times throughout the document. I also used many other acronims. It seems that instead of including a long list of page numbers for the acronim X, I could instead say in a paragraph above an index, but preferably in a footnote to an index something along the lines that "as the topic of the document is X and it is mentioned too many times, it is not included in the index".

Would appreciate help changing the MWE below to print a footnote below the index (preferred), or a paragraph of text above the index.

\documentclass{memoir}
\usepackage{makeidx}
\makeindex
\begin{document}

This document is about X. But it also mentions Y\index{Y}, Z\index{Z}, as well as A\index{A}, B\index{B} and C\index{C}.

% As this document is about X and it is used too many times throughtout the text, it is not included in the index.
\printindex
\end{document}