improve documentation of `FilteredDataset$get_call`

This saegment is very unclear, please rewrite.
```
#' @description
#' Gets a filter expression.
#'
#' This function returns filter calls equivalent to selected items
#' within each of `filter_states`. Configuration of the calls is constant and
#' depends on `filter_states` type and order which are set during initialization.
#'
#' @param sid (`character`)
#' when specified, the method returns code containing conditions calls of
#' `FilterState` objects with `sid` different to this `sid` argument.
```

Start at the documentation for `FilteredDataset$get_call`, especially the `@description` and `sid` parameter text shown in the issue. Rewrite this section so the method's returned filter calls, `filter_states` ordering and type, and the effect of `sid` are clear. Done means the documentation accurately explains the method without ambiguity.

Written by the indexing model from the issue text.