Oddly, urxvt(7) "should be" complete, and it points to the "latest version" at http://pod.tst.eu/http://cvs.schmorp.de/rxvt-unicode/doc/rxvt.7.pod but neither contains the relevant text.
For that, it seems you have to read the sources. In the manual page (not the pod-file), you may read
XTerm Operating System Commands "ESC ] Ps;Pt ST" Set XTerm Parameters. 8-bit ST: 0x9c, 7-bit ST sequence: ESC \ (0x1b, 0x5c), backwards compatible terminator BEL (0x07) is also accepted. any octet can be escaped by prefixing it with SYN (0x16, ^V).
and
Ps = 720 Move viewing window up by Pt lines, or clear scrollback buffer if Pt = 0 (Compile frills). Ps = 721 Move viewing window down by Pt lines, or clear scrollback buffer if Pt = 0 (Compile frills).