Brings parity with the changes for the Media Views introduced in [44900].
- moves the alt text field to the top as first 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 and indentation
Props chetan200891.
Fixes#46875.
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git-svn-id: http://core.svn.wordpress.org/trunk@44967 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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