I'm personally interested in cases where the product is especially complicated, but let's keep the discussion explicitly general.
PHP has really good documentation, compared to say, JS. On the other extreme,
Android has the worst, so bad that the unofficial documentation is Stack Overflow and this: https://guides.codepath.com/android
Parse's has a few holes in it. But overall I like it because it highlights lesser known features like their many-to-many data structures and security ACL, and straight up tells you how to use them without being too wordy. https://docs.parseplatform.org/android/guide/