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.

Built from https://develop.svn.wordpress.org/trunk@45158


git-svn-id: http://core.svn.wordpress.org/trunk@44967 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.2-beta2-45157';
+$wp_version = '5.2-beta2-45158';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.