[except.handle] Add index entry for 'active handler'. (#1595)

Author: Eelis

source/exceptions.tex

@@ -586,8 +586,8 @@
 of the same thread.
 
 \pnum
-A handler is considered active when initialization is complete for
-the parameter (if any) of the catch clause.
+A handler is considered \defnx{active}{exception handling!handler!active} when
+initialization is complete for the parameter (if any) of the catch clause.
 \begin{note}
 The stack will have been unwound at that point.
 \end{note}