Update name and a.k.a of picture-in-picture

Since there's a document picture-in-picture feature and this API is
video-specific, it makes sense to call this video picture-in-picture.

Also add the common PiP shorthand to the description.

Author: foolip

feature-group-definitions/picture-in-picture.yml

@@ -1,5 +1,5 @@
-name: Picture-in-picture
-description: The picture-in-picture API allow websites to create a floating, always-on-top video window.
+name: Video picture-in-picture
+description: The picture-in-picture API allow websites to create a floating, always-on-top video window. Also known as PiP.
 spec: https://w3c.github.io/picture-in-picture/
 caniuse: picture-in-picture
 usage_stats: https://chromestatus.com/metrics/feature/timeline/popularity/2444