[class.friend] Add \grammarterm for elaborated-type-specifier in comment (cplusplus#9036)

Author: Andreas-Krug

source/classes.tex

@@ -4940,7 +4940,7 @@
 class X2 {
   friend Ct;                    // OK, \tcode{class C} is a friend
   friend D;                     // error: \tcode{D} not found
-  friend class D;               // OK, elaborated-type-specifier declares new class
+  friend class D;               // OK, \grammarterm{elaborated-type-specifier} declares new class
 };
 
 template <typename ... Ts> class R {