Rdo_piecetag gives the "Rd_tag" of the Rd macro name.
Rdo_sectype gives the "Rd_tag" of the Rd section x.
is_Rdsecname(name) returns TRUE if name is the name of a
top level Rd section.
The information returned by these functions is obtained from the
charater vectors Rdo_piece_types and
Rdo_predefined_sections.