checksquare should error for generic arrays

Currently

1. it works for more than two dimensions,
2. regardless of what the trailing sizes are

```julia
julia> LinearAlgebra.checksquare(zeros(1,1,1))
1

julia> LinearAlgebra.checksquare(zeros(1,1,2))
1
```

Incidentally, the docstring should document that the error thrown is a `DimensionError`.