Document that wp_before_creating_autosave is be to added in Trac Ticket 20299

[grappler]

It may be better to only add wp_before_creating_autosave once core-#20299 is closed and the filter is in core.

wp-post-meta-revisions/wp-post-meta-revisions.php

Line 36 in ca45992

add_action( 'wp_before_creating_autosave', array( $this, 'wp_autosave_post_revisioned_meta_fields' ) );