Documentation: suggest MAKE_OPAQUE be in a common header

Author: jagerman

docs/advanced/cast/stl.rst

@@ -150,10 +150,10 @@ the declaration
 before any binding code (e.g. invocations to ``class_::def()``, etc.). This
 macro must be specified at the top level (and outside of any namespaces), since
 it instantiates a partial template overload. If your binding code consists of
-multiple compilation units, it must be present in every file preceding any
-usage of ``std::vector<int>``. Opaque types must also have a corresponding
-``class_`` declaration to associate them with a name in Python, and to define a
-set of available operations, e.g.:
+multiple compilation units, it must be present in every file (or in a common
+header) preceding any usage of ``std::vector<int>``. Opaque types must also
+have a corresponding ``class_`` declaration to associate them with a name in
+Python, and to define a set of available operations, e.g.:
 
 .. code-block:: cpp