Add help text to width/height field to explain fallback behaviour

Problem/Motivation

The width/height value are no longer required fields in the Cloudflare Stream formatter.
In #3162183: Do not require width & height settings we created a fallback that will add a height and/or width of 100% to the video if no value was added.
This isn't documented or shown to the developer/sitebuilder.

Proposed resolution

Add the needed help text which explains this behaviour.

Remaining tasks

Create patch