For a lower threshold, prepDocuments will drop words which appear in
fewer than that number of documents, and remove documents which contain no
more words. This function allows the user to pass a vector of lower
thresholds and observe how prepDocuments will handle each threshold.
This function produces three plots, showing the number of words, the number
of documents, and the total number of tokens removed as a function of
threshold values. A dashed red line is plotted at the total number of
documents, words and tokens respectively.