ps.1: Clarify drs, cf. top.1 on DATA

top.1 clarifies that drs isn't the amount of physical memory currently in used/mapped: https://gitlab.com/procps-ng/procps/-/blob/newlib/top/top.1#L746 The previous phrasing could be interpreted as drs being a subset of drs.

Note: I didn't yet run roff on this, so please check whether I did any markup mistakes while aligning those two texts.