[expr.const] Add missing definition of 'usable in constant expressions'

for (sub)objects and reference members.

source/expressions.tex

@@ -6527,10 +6527,16 @@
 
 \pnum
 A variable is
-\defnx{usable in constant expressions}{usable in constant expression} after
+\defn{usable in constant expressions} after
 its initializing declaration is encountered if it is a constexpr variable, or
 it is of reference type or of const-qualified integral or enumeration type, and
 its initializer is a constant initializer.
+An object or reference is \defn{usable in constant expressions}
+if it is a variable that is usable in constant expressions,
+a template parameter object\iref{temp.param},
+a string literal object\iref{lex.string},
+or a non-mutable subobject or reference member
+of an object that is usable in constant expressions.
 
 \pnum
 An expression \tcode{e} is a \defnadj{core constant}{expression}