Improve docs for `rating` parameter

[sindresorhus]

In the search endpoint docs, the rating parameter should document exactly what the allowed values are and what format they should be in. It's currently unclear. There's an example for g, but it's not clear whether G is supported as well and whether PG-13 should be pg-13 or pg13, and what exactly all the allowed values are.

[shwetaatgiphy]

This was super valuable feedback for us @sindresorhus We made the changes to our documentation recently to better document this. Hope you find it helpful.