Document the $update_results parameter in auto-update email filters

[SergeyBiryukov]

In a similar way to #51126, the structure of the $update_results parameter added to auto-update email filters in [48888] should be documented.

The filters in question:

• auto_plugin_update_send_email
• auto_theme_update_send_email

[costdev]

Examples

Plugin:

array (
  0 => (object) array(
     'item' => (object) array(
       'id' => 'w.org/plugins/akismet',
       'slug' => 'akismet',
       'plugin' => 'akismet/akismet.php',
       'new_version' => '5.0.2',
       'url' => 'https://wordpress.org/plugins/akismet/',
       'package' => 'https://downloads.wordpress.org/plugin/akismet.5.0.2.zip',
       'icons' => array(
         '2x' => 'https://ps.w.org/akismet/assets/icon-256x256.png?rev=2818463',
         '1x' => 'https://ps.w.org/akismet/assets/icon-128x128.png?rev=2818463',
       ),
       'banners' => array (
         '1x' => 'https://ps.w.org/akismet/assets/banner-772x250.jpg?rev=479904',
       ),
       'banners_rtl' => array(),
       'requires' => '5.0',
       'tested' => '6.1.1',
       'requires_php' => '5.2',
       'current_version' => '5.0.1',
    ),
     'result' => true,
     'name' => 'Akismet Anti-Spam',
     'messages' => array(
      0 => 'Updating plugin: Akismet Anti-Spam',
      1 => 'Downloading update from https://downloads.wordpress.org/plugin/akismet.5.0.2.zip…',
      2 => 'The authenticity of akismet.5.0.2.zip could not be verified as no signature was found.',
      3 => 'Unpacking the update…',
      4 => 'Installing the latest version…',
      5 => 'Removing the old version of the plugin…',
      6 => 'Plugin updated successfully.',
    ),
  ),
)

Theme:

array (
  0 => (object) array(
     'item' => (object) array(
       'theme' => 'neve',
       'new_version' => '3.4.9',
       'url' => 'https://wordpress.org/themes/neve/',
       'package' => 'https://downloads.wordpress.org/theme/neve.3.4.9.zip',
       'requires' => '5.4',
       'requires_php' => '7.0',
       'current_version' => '3.4.8',
     ),
     'result' => true,
     'name' => 'Neve',
     'messages' => array(
      0 => 'Updating theme: Neve',
      1 => 'Downloading update from https://downloads.wordpress.org/theme/neve.3.4.9.zip…',
      2 => 'The authenticity of neve.3.4.9.zip could not be verified as no signature was found.',
      3 => 'Unpacking the update…',
      4 => 'Installing the latest version…',
      5 => 'Removing the old version of the theme…',
      6 => 'Theme updated successfully.',
    ),
  ),
)

This PR documents the $update_results parameter for the following auto-update email filters:

• auto_plugin_update_send_email
• auto_theme_update_send_email

Trac ticket: https://core.trac.wordpress.org/ticket/51167