* Brackets and postfix= in `@param` add undefined
Previously they only added optionality.
Note that, unlike Typescript, when a parameter initializer is specified
in jsdoc, it does not remove undefined in the *body* of the function.
That's because TS will generate initialisation code, but JS won't, so
the author will have to manually write code to remove undefined from the
type.
```js
/** @param {number} [a=101] */
function f(a) {
// a: number | undefined here
if (!a) {
a = 101
}
// a: number here
}
```
Note that we don't check that
1. the initializer value is actually assigned to the parameter.
2. the initializer's type matches the declared type of the parameter.
Pretty much we just parse it and leave it alone.
* Address PR comments