Matrix p-norm definition in docs is ambiguous

[marcusps]

In the docs, the matrix norms defined in base are described simply as p norms. Unfortunately, the nomeclature and the notation is ambiguous. It is unclear if Julia implements Holder p-norms (matrix norm induced by the vector p-norm) or Schatten p-norms (the vector p-norm of the singular values of the matrix). Which p-norm one expects depends on their field of study (I myself was expecting Schatten p-norms). It turns out Julia implements Holder p-norms.

I don't think the default needs to change, but clarifying the docs would be nice, at least where matrix norms are mentioned explicitly.

[timholy]

Would you be willing to edit the section in base/docs/helpdb.jl? See https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md#improving-documentation

[jiahao]

Related: #41

[marcusps]

Yep, I'll submit a pull request.