Add <slash-type> to main <slash> example

[mscuthbert]

This would make the example at
https://www.w3.org/2021/06/musicxml40/musicxml-reference/examples/slash-element/ less mysterious.

Also explain that the display configuration is only for use-stems="yes"

[mdgood]

Please don't do this. It is unidiomatic.

[mscuthbert]

Can you explain what is unidiomatic? Is it that in a straightforward example like 4/4 we shouldn't put <slash-type> in? Or that there is a correct and universal interpretation for time signatures like 7/8?