Add documentation about boxed_inline to the book

book/src/config_api.md

@@ -259,17 +259,26 @@ found, based on an existing implementation of `get_type`. Otherwise no record
 implementation can be generated.
 
 Some boxed types are passed as `out` parameters to functions and the caller is
-required to allocate them. For this it is necessary to provide Rust
+required to allocate them. For this the boxed type has to be configured as
+inline-allocated boxed type, i.e. the memory of is not allocated on the heap
+but directly as part of the value e.g. on the stack.
+
+For inline-allocated boxed types it is possible to provide Rust
 expressions in the configuration for initializing newly allocated memory for
-them, and to free any resources that might be stored in values of that boxed
-types. By default the memory is zero-initialized and it is valid to provide an
-empty closure like below.
+them, to copy from one value into another one, and to free any resources that
+might be stored in values of that boxed types.
+
+By default the memory is zero-initialized and copying is done via
+`std::ptr::copy()`. If the boxed type contains memory that needs to be freed
+then these functions must be provided.
 
 ```toml
 [[object]]
 name = "Gtk.TreeIter"
 status = "generate"
+boxed_inline = true
 init_function_expression = "|_ptr| ()"
+copy_into_function_expression = "|dest, src| { std::ptr::copy(src, dest, 1); }"
 clear_function_expression = "|_ptr| ()"
 ```