Broadcast clarification responses

Contest_API.md

@@ -1820,6 +1820,9 @@ Properties of clarification message objects:
 
 The recipients of a clarification are the union of `to_team_ids` and `to_group_ids`.  A clarification is sent to all teams if `from_team_id`, `to_team_ids` and `to_group_ids` are null.  Note that if `from_team_id` is not `null`, then both `to_team_ids` and `to_group_ids` must be `null`. That is, teams cannot send messages to other teams or groups.
 
+Clarifications between a team and the jury are typically private. If the jury replies to a clarification and chooses to include additional recipients,
+then in order to preserve referential integrity the `reply_to_id` should be removed for everyone who couldn't see the original message.
+
 #### Modifying clarifications
 
 To add a clarification, clients can use the `POST` method on the clarifications endpoint or the

Contest_Control_System_Requirements.md

@@ -640,7 +640,11 @@ following operations:
 - Determine, for any specific clarification request, what answer was returned to
   the team if the clar has already been answered.
 - Compose an answer to the clar and send it, along with the text of the original
-  clarification request, to the team.
+  clarification request, to the team. The best practice is to include the
+  original clarification text in its entirety, prepending every line by `> `
+  (also known as internet-style quoting). This allows the judge
+  to respond to multiple points or edit it, and doesn't require the original
+  clarification text to understand the response.
 - Optionally choose to also send the clarification request text and answer to
   all teams in the contest.
 - Change the category of a clarification request.