message: add documentation for atom format
diff: @@ -24,6 +24,8 @@ This endpoint accepts the following query-string parameters:
  rows are represented as arrays of 2-element [column, value] arrays instead
  of objects.
  - All values for each cell are returned, without timestamps
  + - `atom`: a single atomic value, without the row name or the column name
  + - The query will fail if anything else than a single row / column is returned.
  - `headers`: boolean (default `true`), if `true` the table format will include a header.
  - `rowNames`: boolean (default `true`), if `true` an implicit column called `_rowName` will
  be added, containing the row name.
  