Document Steepfile DSL

[ParadoxV5]

For example, what’s a target?
I’ve been rocking target :'.' since I started verifying Steep bugs, but not until #1387’s diffs surfacing internals did I realize that I’m not supposed to do that.

steep/lib/steep/cli.rb

Lines 130 to 133 in 1b9b304

	opts.on("--group=GROUP", "Specify target/group name to type check") do |arg|
	# @type var arg: String
	target, group = arg.split(".")
	target or raise

target '_' it is then.