[util.smartptr.atomic] Add cross-reference to <memory> header (#5207)

Author: jensmaurer

source/atomics.tex

@@ -2735,6 +2735,9 @@
 \pnum
 The library provides partial specializations of the \tcode{atomic} template
 for shared-ownership smart pointers\iref{smartptr}.
+\begin{note}
+The partial specializations are declared in header \libheaderref{memory}.
+\end{note}
 The behavior of all operations is as specified in \ref{atomics.types.generic},
 unless specified otherwise.
 The template parameter \tcode{T} of these partial specializations