Add subreddit restriction parameters to SubredditModeration.update() doc string
Add subreddit restriction parameters to SubredditModeration.update() doc string

* `disable_contributor_requests`
* `restrict_commenting`
* `restrict_posting`

gschizas/praw,praw-dev/praw,praw-dev/praw,leviroth/praw,gschizas/praw,leviroth/praw