[class.base.init] Add "direct" for _mem-initializer-id_-named members

A _mem-initializer-id_ cannot be used to initialize a base class
data member from a derived class constructor; therefore, we mean
_direct_ non-static data member.

Author: hubert-reinterpretcast

source/classes.tex

@@ -5524,7 +5524,7 @@
 Unless the
 \grammarterm{mem-initializer-id}
 names the constructor's class,
-a non-static data member of the constructor's class, or
+a direct non-static data member of the constructor's class, or
 a direct or virtual base of that class,
 the
 \grammarterm{mem-initializer}