Document cancellation reason field

The field is added automatically, but wasn't documented

ec_api/apps/api_docs/blueprints/data_structures.apibp

@@ -27,6 +27,12 @@
 + geometry (Point)
 
 
+## CancellationReason (enum)
++ `NO_CANDIDATES`
++ `EQUAL_CANDIDATES`
++ `UNDER_CONTESTED`
++ `CANDIDATE_DEATH`
+
 ## ValidFinderResponse (object)
 + `address_picker`: `false` (boolean) - True if we need to show this user an address picker
 + addresses (array[object]) - An array of address objects containing the addresses applicable to this request (if necessary)
@@ -79,6 +85,7 @@
                 + `elected_role`: `Local Councillor` (string) - Name of the role the winner(s) of this election will assume
                 + metadata: (object, nullable) - Object containing information about special conditions for the user to be aware about (e.g: cancelled elections, voter id pilot). (details TBC)
                 + cancelled: `false` (boolean) - True if this ballot has been cancelled
+                + cancellation_reason: `null` (CancellationReason, nullable) - If a ballot has been cancelled, this key may hold a codified reason for that ballot's cancellation
                 + `replaced_by`: (string, nullable) - If a ballot has been cancelled (cancelled = true) and rescheduled for a later date, this key will hold the ballot_paper_id of the ballot that replaces it.
                 + replaces: (string, nullable) - If this ballot replaces another cancelled ballot, this key will hold the ballot_paper_id of the ballot that it replaces.
                 + `election_id`: `local.cardiff.2017-05-04` (string) - Identifier for this ballot's parent election group

ec_api/apps/api_docs/blueprints/notes.apibp

@@ -65,8 +65,18 @@ information about the pilots.
 
 Sometimes a scheduled poll may be cancelled. This is usuaully either because
 the election is uncontested or due to death of a candidate.
-API responses for a postcode or address in an authority where identification
-is required will include this information in the `notifications` array. e.g:
+
+When a single ballot is cancelled, a `cancellation_reason` will be added to the response. The values are:
+
+* `NO_CANDIDATES`: No candidates
+* `EQUAL_CANDIDATES`: Equal candidates to contested seats
+* `UNDER_CONTESTED`: Fewer candidates than seats
+* `CANDIDATE_DEATH`: Death of a candidate
+
+The fact the election is cancelled _must_ be presented to the user, but it's up to consumers to define if or how the
+reason for the cancellation is presented.
+
+If all elections for a query are cancelled we will add a notification in the `notification` array (see above), e.g:
 
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