Language clarification in extract_diagnostics (#109)

* Language clarification in `extract_diagnostics`

Clarified:

* "These diagnostics are then flagged if they exceed specific thresholds" makes it sound like the returned dataframe flags each __individually__ (which it doesn't, hence suggest change of language)
* "Exceed specific thresholds": sometimes it's if the value is below a threshold hence I suggest reword to "outside acceptable range"

Possible refactors / improvements of this function could be:

<details>

1. Return a flag for each diagnostic rather than for all of them jointly. Would it ever be useful to know in particular which diagnostic was violated (straight away, I know it could be recovered with some digging)
2. Allowing setting of the thresholds. As a user I imagine this might be useful, like if certain conditions mean that diagnostics are over or under flagging. I can see here it's a bit annoying to do and would probably have to be like a user input list of named values.

* Redocument

R/extract_diagnostics.R

@@ -4,8 +4,8 @@
 #' model and provided data. It checks for low case counts and computes
 #' diagnostics from the fitted model, including the mean acceptance
 #' statistic, divergent transitions, maximum tree depth, and Rhat values.
-#' These diagnostics are then flagged if they exceed specific thresholds,
-#' and the results are returned as a data frame.
+#' Additionally, a combined flag is computed indicating if any diagnostics
+#' are outside an acceptable range. The results are returned as a data frame.
 #'
 #' @param fit The model fit object from `EpiNow2`
 #' @param data A data frame containing the input data used in the model fit.
@@ -42,9 +42,9 @@
 #'         counts in the data. See \code{low_case_count_diagnostic()} for more
 #'         information on this diagnostic.
 #'   \item \code{epinow2_diagnostic_flag}: A combined flag that indicates if
-#'         any diagnostic thresholds are exceeded. The diagnostic thresholds
-#'         (1) mean_accept_stat < 0.1, (2) p_divergent > 0.0075, (3)
-#'         p_max_treedepth > 0.05, and (4) p_high_rhat > 0.0075.
+#'         any diagnostic metrics are outside an accepted range, as determined
+#'         by the thresholds: (1) mean_accept_stat < 0.1, (2) p_divergent >
+#'         0.0075, (3) p_max_treedepth > 0.05, and (4) p_high_rhat > 0.0075.
 #' }
 #' @export
 extract_diagnostics <- function(fit,