docs: add likes attribute to Comment and Submission model doc comments

[alythobani]

Feature Summary and Justification

Adds documentation for the likes attribute to praw.models.Submission and praw.models.Comment models.

There's no current open issue for this, although #726 was filed in 2017 that mentioned adding documentation for Submission.likes (among many other fields, some of which have been added already by other PRs, but likes hasn't). #208 is a related issue filed in 2013 by a user who wasn't yet aware of the likes attribute.

References

• https://github.com/reddit-archive/reddit/wiki/JSON#comment-implements-votable--created
• https://github.com/reddit-archive/reddit/wiki/JSON#link-implements-votable--created
• https://github.com/reddit-archive/reddit/wiki/JSON#votable-implementation
• https://www.reddit.com/r/redditdev/comments/2y26ty/praw_python_what_happened_to_the_likes_attribute/
• https://www.reddit.com/r/redditdev/comments/8bomna/how_to_check_if_a_submission_is_already_upvoted/
• Note: Reddit's current API docs don't mention likes, but testing manually shows that it is provided for both Comment and Submission instances, with its value corresponding to actual vote status as described above

[LilSpazJoekp]

Thank you!