Chage return type of method in JDA class. (#2504)

src/main/java/net/dv8tion/jda/api/JDA.java

@@ -1584,6 +1584,9 @@ default PrivateChannel getPrivateChannelById(long id)
  * <br>This will fail with {@link net.dv8tion.jda.api.requests.ErrorResponse#UNKNOWN_USER UNKNOWN_USER}
  * if the user does not exist.
  *
+ * <p>If the channel is cached, this will directly return the channel in a completed {@link RestAction} without making a request.
+ * You can use {@link CacheRestAction#useCache(boolean) action.useCache(false)} to force an update.
+ *
  * <p><b>Example</b><br>
  * <pre>{@code
  * public void sendMessage(JDA jda, String userId, String content) {
@@ -1607,7 +1610,7 @@ default PrivateChannel getPrivateChannelById(long id)
  */
  @Nonnull
  @CheckReturnValue
- default RestAction<PrivateChannel> openPrivateChannelById(@Nonnull String userId)
+ default CacheRestAction<PrivateChannel> openPrivateChannelById(@Nonnull String userId)
  {
  return openPrivateChannelById(MiscUtil.parseSnowflake(userId));
  }