Add documentation of `spec for:`

[LinqLover]

For instance, I'm trying to specify a package that is only loaded in Squeak versions older than Squeak 6.0Alpha. However, this spec baseline always loads the package:

spec for: #'squeak5.x' do: [
		spec package: 'SimulationStudio-Compatibility-Squeak5'.
		spec package: 'SimulationStudio-Base' with: [
			spec includes: 'SimulationStudio-Compatibility-Squeak5']].

(context)

What are the possible values for the #for: parameter and how is decided whether the block is evaluated?