A character vector of variables to collapse text across,
or NULL.
For tokens like n-grams or sentences, text can be collapsed across rows
within variables specified by collapse before tokenization. At tidytext
0.2.7, the default behavior for collapse = NULL changed to be more
consistent. The new behavior is that text is not collapsed for NULL.
Grouping data specifies variables to collapse across in the same way as
collapse but you cannot use both the collapse argument and
grouped data. Collapsing applies mostly to token options of "ngrams",
"skip_ngrams", "sentences", "lines", "paragraphs", or "regex".