docs:(Client): disambiguate the description of channels collection (#3251)

* Disambiguate the description of <client>.channels

Although not explicitly said, the current wording makes it seem like all channels are cached and available at any time in this store. Hopefully this variation makes it a bit clearer.

* make more explicit (I think)

* remove trailing white spaces

src/client/Client.js

@@ -113,7 +113,8 @@ class Client extends BaseClient {
     /**
      * All of the {@link Channel}s that the client is currently handling, mapped by their IDs -
      * as long as sharding isn't being used, this will be *every* channel in *every* guild the bot
-     * is a member of, and all DM channels
+     * is a member of. Note that DM channels will not be initially cached, and thus not be present
+     * in the store without their explicit fetching or use.
      * @type {ChannelStore<Snowflake, Channel>}
      */
     this.channels = new ChannelStore(this);