Document wxStockHelpStringClient used by wxGetStockHelpString()

This should have been done in e80d78c2c9 (Document
wxGetStockHelpString() function, 2026-02-04) as all types used by the
documented functions should themselves be documented.

Closes #26206.

interface/wx/stockitem.h

@@ -81,6 +81,16 @@ wxString wxGetStockLabel(wxWindowID id, long flags = wxSTOCK_WITH_MNEMONIC);
 
 ///@}
 
+/**
+    Used with wxGetStockHelpString() to specify the context in which help
+    string is used.
+ */
+enum wxStockHelpStringClient
+{
+    /// Help string to use for menu items.
+    wxSTOCK_MENU
+};
+
 /** @addtogroup group_funcmacro_misc */
 ///@{