Update scale.md: it's neato and twopi - specific

Only used from neatogen/ and twopigen/

```
> rg \"scale\"
lib/neatogen/adjust.c
948:    ITEM(AM_NSCALE, "scale", "scaling"),
1052:    if ((p = agget(g, "scale"))) {

lib/twopigen/twopiinit.c
103:    if ((s = agget(g, "scale")) && *s) {
```

(I have filtered out false positives: tcl 'scale' command and HTML <img scale> attribute.