[lib.types.movedfrom] Add cross-reference to [defns.valid] (#7365)

Author: Eisenwave

source/lib-intro.tex

@@ -3927,7 +3927,7 @@
 Objects of types defined in the \Cpp{} standard library may be moved
 from\iref{class.copy.ctor}. Move operations may be explicitly specified or
 implicitly generated. Unless otherwise specified, such moved-from objects shall
-be placed in a valid but unspecified state.
+be placed in a valid but unspecified state\iref{defns.valid}.
 
 \pnum
 An object of a type defined in the \Cpp{} standard library may be