Clarify documentation for map.loaded(), map.isStyleLoaded()

mapbox-gl-js version: 0.45.0

Refs https://github.com/mapbox/mapbox-gl-js/issues/6707

The documentation for map.loaded() and map.isStyleLoaded() do not make it clear what the difference is between them.

    /**
     * Returns a Boolean indicating whether the map is fully loaded.
     *
     * Returns `false` if the style is not yet fully loaded,
     * or if there has been a change to the sources or style that
     * has not yet fully loaded.
     *
     * @returns {boolean} A Boolean indicating whether the map is fully loaded.
     */
    loaded()

    /**
     * Returns a Boolean indicating whether the map's style is fully loaded.
     *
     * @returns {boolean} A Boolean indicating whether the style is fully loaded.
     */
    isStyleLoaded()

It should be clear from reading the docs:

• What "fully loaded" means / what might cause it to be not true.
• Under what conditions map.loaded() !== map.isStyleLoaded()