Issue Description:

tldr; The section of custom subscripts should in some fashion discuss the different between a parameter-only subscript and one with a label-parameter.

The subscript section of the manual has numerous examples and even has a "Subscript Syntax" section, where you can see this:

subscript(index: Int) -> Int

Other text says the syntax is similar to Swift method syntax. Fine. But then I scratch my head - in a method, to call the above, I'd have to use a "index" label, say:

let foo = myExamplke[index: 1]

Ah, so I get it - there is an implied "_" label in the above declaration - under the hood its:

subscript(_ index: Int) -> Int

Then, the other day, I see an example of a custom subscript method similar to this:

subscript(withFloat value: Float) -> Int

Now my head is really spinning! The "under the hood" declaration of the above cannot possible be:

subscript(_ withFloat value: Float) -> Int

So then I think I'm getting it. If there is just a single label (the parameter name), then the "_" is implied. If there are two "labels" (a label and parameter), then the label works just like in a method descriptions.

subscript(_ withFloat value: Float) -> Int
let foo = myObject[withFloat: 3.3]

PS: I did look at the "Language Reference", but it doesn't appear that this is covered (although it well may be, that isn't an easy read)