The "Rd_tag" attributes of the top level elements of rdo are
collected in a character vector. Argument nulltag is used for
elements without that attribute. This guarantees that the result is a
character vector.
Rdo_tags is similar to the internal function
tools:::RdTags. Note that tools:::RdTags may return a
list in the rare cases when attribute Rd_tags is not present in
all elements of rdo.