Google has begun updating the official documents for hreflang help. Before this, it only had the contents regarding the use of language annotations, several supported values, and also the general mistakes. The new version for Google traverse through the three ways to indicate the guidelines, alternate pages, supported region or language codes, all use x-default tagging for languages that do not match, debugging methods, and general mistakes.
Aleyda Solis, an SEO, was the first person to notice this change and she mentioned it on her Twitter post.
She said that the new guidelines on expanded and better documentation by using hreflang usage by Google have proved to be really helpful.
See the forum discussion at Twitter.