splitUnit behavior does not match documentation

[Xuerian]

Describe the bug
splitUnit behavior does not match documentation

To Reproduce

1. Go to https://mathjs.org/index.html
2. Evaluate (1m).splitUnit(["ft", "in"]) in the parser

Expected: [ 3 feet, 3.3700787401575 inch ]
Result: [1m]

[gwhitney]

Yes, and even (1m).splitUnit(['cm','um']) produces [1 m], which is also not as expected.

[AHrcTpeM]

What is 1m in js?

[gwhitney]

By default (i.e. if you have not assigned any value to the identifier m), the expression 1m evaluates to a representation of the measured quantity of "one meter" -- a so-called Unit value in mathjs.

[Xuerian]

What is 1m in js?

This is in reference to Math.js expressions: https://mathjs.org/docs/expressions/syntax.html