[dcl.enum] Add index entry for 'fixed' underlying type. (#1667)

Author: Eelis

source/declarations.tex

@@ -2059,7 +2059,7 @@
 The underlying type can be explicitly specified using an \grammarterm{enum-base}.
 For a scoped enumeration type, the underlying type is \tcode{int} if it is not
 explicitly specified. In both of these cases, the underlying type is said to be
-\term{fixed}.
+\defnx{fixed}{type!underlying!fixed}.
 Following the closing brace of an \grammarterm{enum-specifier}, each
 enumerator has the type of its enumeration.
 If the underlying type is fixed, the type of each enumerator