Make it clear the subset flag can be omitted

[SingleAccretion]

It is not necessary to explicitly specify the subset flag when building.

[joperezr]

Hello @SingleAccretion, while I agree that the subset is implicit, this is only the case when it is the first argument so I believe it is intentional that our docs are in the verbose format. If anything, I think we should only mention in the subset parameter that if it is the first item, you can omit it (which we might already have this mention). @ViktorHofer thoughts?

[SingleAccretion]

I suspected as much, yes. I agree that it would probably be better to just point out that the flag can be omitted, and this is indeed mentioned in various places (notably, all examples in ./build -h use the short form).

What lead me to do it this way is the fact that, having pointed a person new to dotnet/runtime to this document (and the directory in general), they did not discover this fact. Not only that, but one of the developers of dotnet/runtime did not know it either, which lead me to believe this knowledge is not as pervasive as I've assumed, which in turn lead to this PR.

[ViktorHofer]

I'm fine with either merging this or not but FWIW I would prefer a consistent style in our docs. I doubt that coreclr's README.md is the only place that uses the explicit subset style.

[SingleAccretion]

Other docs use the explicit style. I will revert the changes and just mention the flag can be omitted.

[ViktorHofer]

FWIW I think the implicit style makes more sense as I don't see a reason why the subset wouldn't be the first param.

[joperezr]

Thanks for the contribution @SingleAccretion !