[structure.specifications] clarify description of Results element

The text "... the expression is an lvalue if the type is an lvalue reference type, an xvalue if the type is an rvalue reference type, and a prvalue otherwise" clearly indicates that a Result element describes the type and value category of an expression, yet we summarize it as only "a description of the type of the expression".

Author: CaseyCarter

source/lib-intro.tex

@@ -394,7 +394,7 @@
 \result
 for a \grammarterm{typename-specifier}, a description of the named type;
 for an \grammarterm{expression},
-a description of the type of the expression;
+a description of the type and value category of the expression;
 the expression is an lvalue if the type is an lvalue reference type,
 an xvalue if the type is an rvalue reference type, and
 a prvalue otherwise.