[dcl.enum] Use more natural and unambiguous phrasing around the

definition of whether an enumeration type's underlying type is fixed.

Author: zygoloid

source/declarations.tex

@@ -1892,9 +1892,9 @@
 \indextext{\idxcode{enum}!underlying~type}%
 Each enumeration defines a type that is different from all other types.
 Each enumeration also has an underlying type.
-The underlying type can be explicitly specified using \grammarterm{enum-base};
-if not explicitly specified, the underlying type of a scoped enumeration type
-is \tcode{int}. In these cases, the underlying type is said to be
+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}.
 Following the closing brace of an \grammarterm{enum-specifier}, each
 enumerator has the type of its enumeration.
@@ -1937,7 +1937,6 @@
 \grammarterm{enum-specifier}, at which point it becomes a complete type.
 
 \pnum
-\indextext{type!enumeration underlying}%
 For an enumeration whose underlying type is not fixed,
 the underlying type
 is an