Build matrix documentation ambiguities

[andy-v-h]

The docs on matrix suggest that there are more than three listed

matrix values into command step attributes such as environment variables, labels and commands

"such as" reads to me like it could interpolate values into queues, for example. This is lightly asked about in #1494 but I don't think there's been a resolution to that.

But that does not seem to be the case? If it's only these three could the verbiage be made more concrete?