Add documentation for all supported functions

[gagern]

Unless an implemented function is deliberately unsupported, it should be documented. This probably isn't the case yet.

• autodiff$3
• compileToWebGL$1
• currentlanguage$0
• drawconic$1
• drawpolygon$1
• dump$1
• eig$1
• errc$1
• fillpolygon$1
• generateWebGL$2
• grey$1
• international$1
• international$2
• kernel$1
• keycode$0
• matrixrowcolumn$1
• para$2
• perp$1
• perp$2
• plotX$1
• randominteger$1
• rank$1
• setsimulationaccuracy$1
• setsimulationquality$1
• setsimulationspeed$1
• setTimeout$2
• timestamp$0
• use$1
• version$0

This list was originally generated automatically using the command

diff -u <(grep ^# ref/*.md | grep -Eo '`([a-zA-Z0-9]+)\([^)`]*\)`' | tr -d \` | perl -pe 's!\(.*\)!$a=$&;"\$".($a eq "()"?0:1+($a=~tr/,//))!e' | sort) <(node -e 'Object.keys(require("rewire")("./build/js/exposed.js").__get__("evaluator")).filter(n=>/\$/.test(n)).forEach(n=>console.log(n))' | sort) | tail -n+3 | grep '^+' | sed 's/^./* [ ] /'

It is the counterpart to #219.