Update recommended MySQL backup and restore commands

[000yesnt]

A while ago, when I had to recover a backup of my Seafile 11.0 database, a few things stood out to me:

• As someone who isn't familiar with MySQL (or database administration in general), I didn't know mysqldump didn't save users unless the mysql database was also saved. It was a little annoying to dig through the documentation for the right command, so I added a note that reminds the reader.
  • For the Docker section, I also added a note stating that creating the seafile user isn't necessary, as the Docker backup command includes mysql
• The mysqldump commands that are shown in the documentation lack certain options that would make the resulting dump more complete. In particular, the lack of --default-character-set=utf8mb4 creates issues with Unicode file names. I included other options that I see frequently recommended online - feedback on whether any of them are needed is appreciated.
• The repeated mysqldump commands can be simplified by using the --databases or --all-databases options.

This PR tries to address these problems.