docs(ChannelManager): fetch can return Promise<null> (#5422)

src/managers/ChannelManager.js

@@ -75,7 +75,7 @@ class ChannelManager extends BaseManager {
    * @param {Snowflake} id ID of the channel
    * @param {boolean} [cache=true] Whether to cache the new channel object if it isn't already
    * @param {boolean} [force=false] Whether to skip the cache check and request the API
-   * @returns {Promise<Channel>}
+   * @returns {Promise<?Channel>}
    * @example
    * // Fetch a channel by its id
    * client.channels.fetch('222109930545610754')