The floatrow package offers the \floatfoot macro for notes in addition to a float's \caption.
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\documentclass{article}
\usepackage[capposition=top]{floatrow}
\begin{document}
\begin{figure}
% \centering% default with `floatrow`
\rule{1cm}{1cm}% placeholder for `\includegraphics`
\caption{A figure}
\floatfoot{A note}
\end{figure}
\end{document}--------------------------------------------------Output:
