No embedded docs for (some?) units

With the restored autocomplete facility in the demo, I discovered that mathjs knows about the units 'candela' and 'angstrom'. As these seem somewhat esoteric, I tried `help(candela)` and `help('candela')` (and similarly for angstrom) in the homepage demo, and all of these calls returned `Error: No documentation found on "candela"`.

There should be (1) a test that all appropriate symbols have embedded docs, and (2) all units should have embedded docs.