docs for showcompact and array printing

[ssfrr]

The current docs for showcompact say:

Show a more compact representation of a value. This is used for printing array elements.

In print_matrix_repr, compactness is determined by array_eltype_show_how(X), which ends up only being compact if isleaftype(T) && (T<:Number || T<:AbstractString)

I wanted to clarify expected behavior. I came across this because I'm defining a non-numeric type that I want to display more compactly when in a vector, and initially thought I could just define showcompact.

Given #14052 this may soon become a moot point, so feel free to say so and close if it's not worth worrying about.

EDIT: I just realized that showcompact is used by writemime when displaying to the REPL, so I'm guessing that's what the note in the docstring is referring to.

[ssfrr]

also, I'm happy to either update the docs or the implementation once I know what the right direction is, and if it's not about to be imminently clobbered.

[JeffBezanson]

The help for showcompact now explains how to deal with it. This is also part of #5709.