Document class_identifier argument of root class

This can prevent mistake in forgetting the "java::" prefix.

Author: romainbrenguier

src/java_bytecode/java_root_class.cpp

@@ -57,7 +57,8 @@ void java_root_class(symbolt &class_symbol)
 /// \param jlo [out] : object to initialize
 /// \param root_type: type of the root class
 /// \param lock: lock field
-/// \param class_identifier: class identifier field
+/// \param class_identifier: class identifier field, generally begins with
+///        "java::" prefix.
 void java_root_class_init(
   struct_exprt &jlo,
   const struct_typet &root_type,