Remove depreciated method from documentation

I was going though the documentation for parsing comments and noticed that the example under "[Getting all recent comments to a subreddit or everywhere](https://praw.readthedocs.org/en/latest/pages/comment_parsing.html#getting-all-recent-comments-to-a-subreddit-or-everywhere)" wasn't working. I did some research and found out via issue #254 the method `get_all_comments()` has been depreciated in the latest version and that using `get_comments('all')` is preferred.

I went ahead and changed the docs in order to accurately represent this so people won't have trouble in the future.