[Document the] secant function messing up sec unit

I believe "sec" is a built-in alias for the time unit seconds. However, if I type "5 sec" into the https://mathnotepad.com (or the evaluate input on https://mathjs.org) the function sec seems to take precedence and throw a TypeError even though it is expecting a unit:

```TypeError: Unexpected type of argument in function multiplyScalar (expected: number or Unit or string or boolean or BigNumber or Complex or Fraction, actual: function, index: 1)```

Can we improve the parser to avoid this confusion? It seems the unit "sec" is currently unavailable.