[dcl.attr.fallthrough] Use \defnadj for "fallthrought statement"

The term "fallthrough statement" should be properly introduced in italic
style text.

Author: frederick-vs-ja

source/declarations.tex

@@ -9780,8 +9780,7 @@
 \pnum
 The \grammarterm{attribute-token} \tcode{fallthrough}
 may be applied to a null statement\iref{stmt.expr};
-\indextext{statement!fallthrough}
-such a statement is a fallthrough statement.
+such a statement is a \defnadj{fallthrough}{statement}.
 No \grammarterm{attribute-argument-clause} shall be present.
 A fallthrough statement may only appear within
 an enclosing \keyword{switch} statement\iref{stmt.switch}.