Do not use @inheritDoc and @api together

Without this patch we get the following in the `ol-externs.js` file:

```js
/**
 */
ol.style.Circle.prototype.getImage = function() {};

/**
 */
ol.style.Icon.prototype.getImage = function() {};

```

which does not correspond to the correct signatures of these two functions.

With this patch we get the correct signature in the externs file:

```js
/**
 * @param {number} pixelRatio
 * @return {HTMLCanvasElement|HTMLVideoElement|Image}
 */
ol.style.Circle.prototype.getImage = function(pixelRatio) {};


/**
 * @param {number} pixelRatio
 * @return {HTMLCanvasElement|HTMLVideoElement|Image}
 */
ol.style.Icon.prototype.getImage = function(pixelRatio) {};
```

Please review.
 
And `@return {Image}` for `ol.style.Icon#getImage`?