Listed here, The MySQL documentation is quite massive. They have guides, workbenches, release notes going back from 8.0 to 7.4.3, and information about basically every form of MYSQL available on the market.
Listed on O'reilly.com, MYSQL is used by many sites like slashdot and Yahoo, handlers that support transactions, multithreading, sophisticated privilege systems, table handlers, table types formatted as files, and much more.
As mentioned above, MYSQL is used for big name sites like Yahoo and Slashdot, but Wikipedia mentions many other Web Applications. It works in tandem with Linux, Apache, and Perl/PHP/Python. It is also used by Drupal, Joomla, phpBB, Wordpress, Facebook, Flickr, MediaWiki, Twitter, and YouTube.
In Summary, MySQL has a massive storage of documentation, for just about every version that is currently in use, as well as every edition, along with other guides and manuals. They are highly recommended by other sites comparing them against other databases, as they are far more feature rich than other relational database systems. They are used by a number of big name websites, like Youtube, Twitter, Wordpress, and many more.