Document use of filter.

src/wp-includes/fonts.php

@@ -142,9 +142,19 @@ function wp_font_dir( $create_dir = true ) {
 /**
  * Returns the font directory for use by the font library.
  *
- * This function is a callback for the 'upload_dir' filter. It is used not
+ * This function is a callback for the {@see 'upload_dir'} filter. It is not
  * intended to be called directly. Use wp_get_font_dir() instead.
  *
+ * The function can be used when extending the font library to modify the upload
+ * destination for font files via the upload_dir filter. The recommended way to
+ * do this is:
+ *
+ * ```php
+ * add_filter( 'upload_dir', '_wp_filter_font_directory' );
+ * // Your code to upload or sideload a font file.
+ * remove_filter( 'upload_dir', '_wp_filter_font_directory' );
+ * ```
+ *
  * @since 6.5.0
  * @access private
  *