docs: include change in docs

Author: henryiii

docs/advanced/cast/custom.rst

@@ -46,7 +46,7 @@ type is explicitly allowed.
              * function signatures and declares a local variable
              * 'value' of type inty
              */
-            PYBIND11_TYPE_CASTER(inty, _("inty"));
+            PYBIND11_TYPE_CASTER(inty, const_str("inty"));
 
             /**
              * Conversion part 1 (Python->C++): convert a PyObject into a inty

docs/upgrade.rst

@@ -17,6 +17,10 @@ v2.9
   converted to using ``py::module_::import("types").attr("SimpleNamespace")``
   instead.
 
+* The use of ``_`` in custom type casters can now be replaced with the more
+  readable ``const_str`` instead. The old ``_`` shortcut has been retained
+  unless it is being used as a macro (like for gettext).
+
 
 .. _upgrade-guide-2.7: