Updated Build Troubleshooting (markdown)

Build-Troubleshooting.md

@@ -61,6 +61,24 @@ MP_OBJ_TYPE_GET_SLOT(type, slot) # equivalent to type->slot (but you must know t
 MP_OBJ_TYPE_GET_SLOT_OR_NULL(type, slot) # marginally less efficient but will return NULL if the slot is not set
 ```
 
+If your code needs to access fields on `mp_obj_type_t` (this should be relatively rare), then you will now need to do so via the helper macros. Code that previously did:
+
+```c
+if (type->call) {}  // See if field is set.
+mp_call_fun_t f = type->call;  // Access field.
+type->call = f;  // Set field.
+```
+
+will need to be changed to:
+
+```c
+if (MP_OBJ_TYPE_HAS_SLOT(type, call)) {}
+mp_call_fun_t f = MP_OBJ_TYPE_GET_SLOT(type, call); // Or MP_OBJ_TYPE_GET_SLOT_OR_NULL(type, call) if you do not know whether the slot is set.
+MP_OBJ_TYPE_SET_SLOT(type, call, f, index);
+```
+
+In the last case, you need to know the index (likely this is being used in a situation where you're initialising a new mp_obj_type_t instance so you can generate the slots sequentially).
+
 ## buffer
 
 The definition for a type with a buffer slot has been simplified and no longer requires a nested struct.