Internal function to make new system-level docvars
make_docvars(n, docname = NULL, unique = TRUE, drop_docid = TRUE)the number of documents
a character vector for the names of documents. Must be the
same length as n or NULL. If NULL, names are generated automatically.
if TRUE, names must be all unique. If FALSE, documents with the same
names are treated as segments from the same document and given serial number.
if TRUE, drop unused names of documents.