docs(asNever): minor edits to docstring

src/internal/types/asNever.ts

@@ -1,9 +1,13 @@
 /**
- * A utility used to typecheck that a symbol has type `never`. Throws if called.
+ * A utility used to typecheck that a symbol has type `never` and therefore the
+ * call site is unreachable. Throws if called.
  *
  * Example: if `a` has type `0 | 1`, you could write `a === 0 ? 'zero' : a === 1
- * ? 'one' : asNever(a)` to make sure that typecheck will fail if the type of
- * `a` changes to say `0 | 1 | 2`.
+ * ? 'one' : asNever(a)` to make sure that all possibilities for `a` have been
+ * exhausted. If the type of `a` changes to say `0 | 1 | 2`, the type of the
+ * argument passed to `asNever` will be inferred as `2`, and this will cause a
+ * typechecking error because the only type assignable to `never` is `never`
+ * itself.
  */
 export const asNever: (arg: never) => never = () => {
   throw 'asNever has been called; this means that if you use TypeScript, typecheck is failing.';