Many users found the attachment URL field confusing: it says "URL" so it may appear like a field meant to paste a URL into.
Also, the Alt text field is the most important one in terms of content, while the Title field needs to be de-emphasized.
- changes the URL field label to "Copy link"
- moves the alt text field to the top as first field
- avoids to set initial focus on the alt text field
- adds an explanatory text with a link pointing to the W3C "alt decision tree" tutorial
- adds `aria-describedby` to target the explanatory text
- adjusts the CSS accordingly
- updates the QUnit index.html file
Props melchoyce, audrasjb, afercia.
Fixes#41612.
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