[structure.specifications] Clarify that 'Mandates' conditions may or may not be SFINAE-able

Author: jensmaurer

source/lib-intro.tex

@@ -591,6 +591,10 @@
 
 \item
 \mandates the conditions that, if not met, render the program ill-formed.
+\begin{note}
+This definition does not specify whether the conditions are
+checked before or after the function is selected by overload resolution.
+\end{note}
 \begin{example}
 An implementation might express such a condition
 via the \grammarterm{constant-expression}