Improvements for the encryption section

[nunesgh]

@schiessle's explanation of the encryption configuration in nextcloud/server#8283 (comment) should be added to the encryption section of the official documentation.

In addition, the command sudo -u www-data php occ encryption:disable-master-key is not documented but should be. Since the master key is enabled by default, this command is essential for admins who wish to enable per-user keys.