Accessibility: Improve the image "Alternative Text" field in the Edit Media screen.

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.


git-svn-id: https://develop.svn.wordpress.org/trunk@45158 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/media.css

@@ -825,6 +825,13 @@ border color while dragging a file over the uploader drop area */
 /*------------------------------------------------------------------------------
   14.2 - Image Editor
 ------------------------------------------------------------------------------*/
+.wp_attachment_details .attachment-alt-text {
+	margin-bottom: 5px;
+}
+
+.wp_attachment_details .attachment-alt-text-description {
+	margin-top: 5px;
+}
 
 .wp_attachment_details label[for="content"] {
 	font-size: 13px;