DSLX array concat is not documented well

`++` working on concat is buried in an example under the "array conversion" subsection talking about converting to bits. https://google.github.io/xls/dslx_reference/#:~:text=fn%20concat_arrays(a%3A%20u2%5B3%5D%2C%20b%3A%20u2%5B3%5D)%20%2D%3E%20u2%5B6%5D%20%7B%0A%20%20a%20%2B%2B%20b%0A%7D

It would be good to document it explicitly in https://google.github.io/xls/dslx_reference/#concat-expression, which currently can be misinterpreted to only apply to bits (many things that work on bits do not work on arrays).

Related: #1619