add one [doc_m] to a given subsection. for use by [doc_c] [doc_i] [doc_o] only. add one [doc_m] with a blank \$2 (will show up as "~unspecified~") y position of top of the parent (as given by outlet 1 of [doc_layout]) subsection id, such as "c0", "i3" or "oany".