Update numeric.md

[soerenwolfers]

This is part one - document the expected behavior - of my dissection of duckdb/duckdb#14239, which I closed for being too chaotic.

[szarnyasg]

Thanks!

[soerenwolfers]

when to use narrow and when monospace?

[szarnyasg]

I don't have a 100% consistent rule yet. I applied monospace because I thought it will apply to the headers here... but I was wrong. Will try to come up with a recommendation in the contributor guide.