Move global $wp_embed and document its usage.

src/wp-includes/block-template-utils.php

@@ -1158,9 +1158,13 @@ function get_block_file_template( $id, $template_type = 'wp_template' ) {
  *
  * @since 5.9.0
  *
+ * @global WP_Embed $wp_embed An instance of WP_Embed for embedding rich media.
+ *
  * @param string $part The block template part to print. Use "header" or "footer".
  */
 function block_template_part( $part ) {
+	global $wp_embed;
+
 	$template_part = get_block_template( get_stylesheet() . '//' . $part, 'wp_template_part' );
 	if ( ! $template_part || empty( $template_part->content ) ) {
 		return;
@@ -1175,7 +1179,6 @@ function block_template_part( $part ) {
 	$content = do_shortcode( $content );
 
 	// Handle embeds for block template parts.
-	global $wp_embed;
 	$content = $wp_embed->autoembed( $content );
 
 	echo $content;