[heycam/webidl] What's the defference in "number of nullable member type" and 'includes a nullable type"? (#735)

Now we define [**number of nullable member types**](https://heycam.github.io/webidl/#dfn-number-of-nullable-member-types) in union types, and [**includes a nullable type**](https://heycam.github.io/webidl/#dfn-includes-a-nullable-type) in general types.

Currently, **number of nullable member types** is practically referred in the definition of **includes a nullable type**.  IIUC, "a union type `A` includes a nullable type" means "`A`'s number of nullable member types is 1".

So what I wonder is if we need to define **number of nullable member types**. As I describe above, it's quite similar to **includes a nullable type**, and **includes a nullable type** does not require **number of nullable member types** in its definition.
A small difference in their definition seems **includes a nullable type** assumes annotated types whose inner type is nullable.

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