Google Reorganizes Sitemaps Documentation
I believe Google changed the sitemaps-related search developer documentation last week. I am not sure if any of the content specifically changed, but Google did rename some of the pages and create new pages, maybe to consolidate the content and make it easier to understand and access.
Google also added a new section named combining sitemap sections. Google also replaced the “split up your large sitemaps” with “manage sitemaps with sitemap index file.” I did not dig through each file to see exactly what changed but you may want to.
Here is a side-by-side of the old structure versus the new structure, with the added “combining sitemap sections” in the sidebar:
Forum discussion at Twitter.