[smartptr.adapt] Emphasize that casting might not always be viable

Author: jensmaurer

source/utilities.tex

@@ -12162,7 +12162,7 @@
 \pnum
 \begin{note}
 \tcode{reinterpret_cast<void**>(static_cast<Pointer*>(*this))}
-can be a viable implementation strategy.
+can be a viable implementation strategy for some implementations.
 \end{note}
 \end{itemdescr}
 
@@ -12400,7 +12400,7 @@
 \pnum
 \begin{note}
 \tcode{reinterpret_cast<void**>(static_cast<Pointer*>(*this))}
-can be a viable implementation strategy.
+can be a viable implementation strategy for some implementations.
 \end{note}
 \end{itemdescr}