Documentation for `date_to_rfc822` and `uri_escape`

I was adding an RSS feed to a site I was building and was manually using `{{ post.date | date: "%a, %d %b %Y %H:%M:%S %z" }}`, so I decided to add a filter like `date_to_rfc822` to Jekyll. Then I noticed it was already implemented in #892, so I documented it on the site.

While I was at it I added documentation for `uri_escape` to the site and to the source. Now all of the filters Jekyll adds to Liquid are documented on the site and in the source files.