Fix the Privacy Policy help notice (#11999)

* Display the privacy policy help notice with the `admin_notices` action hook.

The new editor does not support the `edit_form_after_title` action hook. Because WordPress Core uses this hook to output the notice, it is not printed to the screen.

After #11604 is merged, legacy style admin notices (`<div class="notice">...notice content...</div>`) will be consumed by the Notices API and displayed. This change ensures that when #11604 is merged the privacy policy notice will appear again when editing the privacy policy page.

* Use `get_post()` instead of `global $post`.

* Add missing `@since` documentation tag.

* Only display the notice if the function is actually hooked to `edit_form_after_title`.

This prevents the notice from displaying twice if the notice has already been moved to the `admin_notices` hook (as would happen in https://core.trac.wordpress.org/attachment/ticket/45057/45057.diff).

lib/compat.php

@@ -116,7 +116,6 @@ function gutenberg_wpautop( $content ) {
 remove_filter( 'the_content', 'wpautop' );
 add_filter( 'the_content', 'gutenberg_wpautop', 6 );
 
-
 /**
  * Check if we need to load the block warning in the Classic Editor.
  *
@@ -304,3 +303,22 @@ function gutenberg_warn_classic_about_blocks() {
 		</script>
 	<?php
 }
+
+/**
+ * Display the privacy policy help notice.
+ *
+ * In Gutenberg, the `edit_form_after_title` hook is not supported. Because
+ * WordPress Core uses this hook to display this notice, it never displays.
+ * Outputting the notice on the `admin_notices` hook allows Gutenberg to
+ * consume the notice and display it with the Notices API.
+ *
+ * @since 4.5.0
+ */
+function gutenberg_show_privacy_policy_help_text() {
+	if ( is_gutenberg_page() && has_action( 'edit_form_after_title', array( 'WP_Privacy_Policy_Content', 'notice' ) ) ) {
+		remove_action( 'edit_form_after_title', array( 'WP_Privacy_Policy_Content', 'notice' ) );
+
+		WP_Privacy_Policy_Content::notice( get_post() );
+	}
+}
+add_action( 'admin_notices', 'gutenberg_show_privacy_policy_help_text' );