[doc] add reference to ENABLED_IF directive in devman

doc/developer/advance.tex

@@ -570,6 +570,12 @@ modify or disable test cases.
 & \textit{None}
 \\
 \hline
+\texttt{ENABLED\_IF}\nscodeidxdef{Test!Directive}{ENABLED\_IF}
+& Conditionally enable subsequent tests, using Dune variables
+  (e.g. \verb|%{bin-available:gcc}|).
+& \texttt{true}
+\\
+\hline
 \texttt{\underline{EXECNOW}}\nscodeidxdef{Test!Directive}{EXECNOW}
 & Run a custom command.
 & \textit{None}