doc: defproc for min*, max*

[bennn]

these have been available but undocumented for years

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I keep rediscovering them by requiring plot/utils and a min* / max* that expect lists. Adding this entry to the docs will make it easier to learn that plot/utils is the exporter.

[alex-hhh]

I'll merge this, since it is just a documentation update, but perhaps min* and max* should be outside of the plot package, defined together with min and max