fix(channel): member delete acts as leave; add coverage and docs

docs/api-reference.md

@@ -802,6 +802,14 @@ Response: `204`
 Auth required.  
 Response: `204`
 
+Behavior:
+
+- `saved messages`: clears personal history only (chat is not removed).
+- `channel` + role `owner/admin`: deletes channel for all members.
+- `channel` + role `member`: acts as leave channel (removes only current membership).
+- `group/private` with `for_all=false`: removes chat for current user only.
+- `for_all=true` (where allowed): deletes chat globally.
+
 ### POST `/api/v1/chats/{chat_id}/clear`
 
 Clear chat history for current user.