Google has recently updated their official documentation for hreflang help. Prior, it only had content around using language annotations, supported language values and a couple of common mistakes. The new version goes through the three methods for indicating your alternate pages, guidelines, supported language/region codes, use the x-default tag for unmatched languages, common mistakes and debugging techniques.
You can see the old version on the wayback machine and compare it to the new version over here.
Aleyda Solis, an international SEO, was the first to notice and post about it on Twitter from what I can tell. She wrote:
There’s a quite nice, expanded and updated official Google documentation about hreflang usage, with guidelines and resources, check it out: https://t.co/NwXH9UsGJD 👏👩🏻💻✈️Also, thanks for including my hreflang generator 🙌😊 pic.twitter.com/87SGXlRFSa
— Aleyda Solis (@aleyda) July 5, 2018
So those of you that have or need to implement hreflang, check out the new docs.
Forum discussion at Twitter.