documentation

[ZBXNEXT-7578] described API changes related to headers, query_fields

* commit '7e0b0fc67f5f9f530e600fde03dcc9f78daa8f5c':
  [ZBXNEXT-7578] removed headers, query_fields from additional item, item prototype fields
  [ZBXNEXT-7578] removed repeats and "can be empty" notes; misplaced change-note
  [ZBXNEXT-7578] described changes for item, item prototype, LLD rule headers and query_fields properties to array of objects