docs: document DidHistory meta fields for billing_cycles_count_changed

Add docstring documenting that DidHistory records carry meta["from"] and
meta["to"] integers when action is "billing_cycles_count_changed", and
that those keys are absent for every other action.

Author: Fivell

src/didww/resources/did_history.py

@@ -2,6 +2,23 @@
 
 
 class DidHistory(DidwwApiModel):
+    """DID History event resource.
+
+    Attributes:
+        did_number (str): The DID number this history event relates to.
+        action (str): The action that occurred (e.g. "billing_cycles_count_changed").
+        method (str): The method used for the action.
+        created_at (datetime): When the event occurred.
+
+    Meta fields (present in the JSON:API ``meta`` hash on the response,
+    not as resource attributes):
+
+        meta["from"] / meta["to"] (int):
+            Only present when action is "billing_cycles_count_changed".
+            The previous (from) and new (to) billing_cycles_count values.
+            Absent for every other action.
+    """
+
     did_number = SafeAttributeField("did_number")
     action = SafeAttributeField("action")
     method = SafeAttributeField("method")