[meta.reflection.queries] Add missing "Constant When" element of "dealias", converted to "Throws".

This seems to have been omitted during application of P2996R13 by
accident.

Author: tkoeppe

source/meta.tex

@@ -4817,6 +4817,11 @@
 static_assert(dealias(^^Y) == ^^int);
 \end{codeblock}
 \end{example}
+
+\pnum
+\throws
+\tcode{meta::exception} unless
+\tcode{r} represents an entity.
 \end{itemdescr}
 
 \indexlibraryglobal{has_template_arguments}%
@@ -4837,7 +4842,7 @@
 \pnum
 \throws
 \tcode{meta::exception} unless
-\tcode{r} represents an entity.
+\tcode{has_template_arguments(r)} is \tcode{true}.
 \end{itemdescr}
 
 \indexlibraryglobal{template_of}%