replace in the header of a document a paragraph containing a bookmark by a flextable. A bookmark will be considered as valid if enclosing words within a paragraph; i.e., a bookmark along two or more paragraphs is invalid, a bookmark set on a whole paragraph is also invalid, but bookmarking few words inside a paragraph is valid.
headers_flextable_at_bkm(x, bookmark, value)
an rdocx object
a flextable object