docs: make clarification about plots in report

Author: atrigila

assets/differentialabundance_report.Rmd

@@ -546,6 +546,7 @@ contrasts_to_print$model <- sapply(contrasts_to_print$Id, function(id) {
 
 print( htmltools::tagList(datatable(contrasts_to_print, caption = paste0("Table of contrasts"), rownames = FALSE, options = list(dom = ifelse(nrow(contrasts_to_print) > 10, 'tp', 't'))) ))
 ```
+Note: For formula-based contrasts without a 'variable', the column was automatically filled with `params$report_grouping_variable` to support grouping in plots and reports.
 
 # Results