Augment the comment of lookup_face.

src/xfaces.c

@@ -4864,7 +4864,13 @@ uncache_face (c, face)
    of frame F.  The face will be used to display character C.  Value
    is the ID of the face found.  If no suitable face is found, realize
    a new one.  In that case, if C is a multibyte character, BASE_FACE
-   is a face for ASCII characters that has the same attributes.  */
+   is a face for ASCII characters that has the same attributes.
+
+   When this function is called from face_for_char (in this case, C is
+   a multibyte character), a fontset of a face returned by
+   realize_face is not yet set, i.e. FACE_SUITABLE_FOR_CHAR_P (FACE,
+   C) is not sutisfied.  The fontset is set for this face by
+   face_for_char later.  */
 
 INLINE int
 lookup_face (f, attr, c, base_face)