docs: remove 'd' from duration values spec

[tpaschalis]

Fixes #599

This PR addresses a spec inconsistency. This is not a breaking change, since d was never actually supported as a valid duration unit.

I still think that the Go team had a good rationale for not adding support for it in time.ParseDuration; if it's so important for people to use higher values we could find a way to support richer expressions such as 5 * 24h in the future instead.

[tpaschalis]

cc @rfratto for visibility, I'll go ahead and merge this if there's no objections.

[rfratto]

since d was never actually supported as a valid duration unit.

To be precise, Alloy/Flow mode has never supported it, but Prometheus and static mode of Grafana Agent do thanks to github.com/prometheus/common/model.Duration

Personally I find most people mean 1d to be 24h, even if sometimes a day is not exactly 24h. I'm ok with us fixing the docs since no component uses model.Duration, but we should double check that the config converters handle converting 1d from Prometheus to 24h in Alloy properly (they should, but wouldn't hurt to check).

[tpaschalis]

Thanks for the feedback! I've verified that the converters do handle days properly (1d gets converted to 24h0m0s).

I'm merging this.