Docs Formatting: Backtick-escape inline code for all dynamic hook docs in wp-admin/*.

Also includes some changes to move hook docs to directly precede hook lines. This is necessary to prevent DocBlock-matching confusion when core is parsed.

Affects DocBlocks for the following hooks:
* `wp_ajax_ . $_REQUEST['action']`
* `wp_ajax_nopriv_ . $_REQUEST['action']`
* `admin_footer- . $GLOBALS['hook_suffix']`
* `admin_head-$hook_suffix`
* `admin_post_nopriv_{$action}`
* `admin_post_{$action}`
* `load-  . $page_hook`
* `load- . $plugin_page`
* `load-importer- . $importer`
* `load- . $pagenow`
* `admin_action_ . $_REQUEST['action']`
* `async_upload_{$type}`
* `add_meta_boxes_ . $post_type`
* `{$taxonomy}_pre_edit_form`
* `{$taxonomy}_term_edit_form_tag`
* `{$taxonomy}_edit_form_fields`
* `{$taxonomy}_edit_form`
* `after-{$taxonomy}-table`
* `{$taxonomy}_pre_add_form`
* `{$taxonomy}_term_new_form_tag`
* `{$taxonomy}_add_form_fields`
* `{$taxonomy}_add_form`
* `media_upload_$type`
* `media_upload_$tab`
* `install_plugins_pre_$tab`
* `install_plugins_$tab`
* `install_themes_pre_{$tab}`
* `install_themes_{$tab}`
* `update-core-custom_{$action}`
* `update-custom_{$action}`
* `user_{$name}_label`

See #30552.


git-svn-id: https://develop.svn.wordpress.org/trunk@30649 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/update.php

@@ -261,7 +261,7 @@ if ( isset($_GET['action']) ) {
 		/**
 		 * Fires when a custom plugin or theme update request is received.
 		 *
-		 * The dynamic portion of the hook name, $action, refers to the action
+		 * The dynamic portion of the hook name, `$action`, refers to the action
 		 * provided in the request for wp-admin/update.php. Can be used to
 		 * provide custom update functionality for themes and plugins.
 		 *