[SSO71] Document supported database migration between sso 70 and 71

From current internal upgrading documentation is not quite clear which steps should user make for database migration.

Sso offers two ways:

• update - this is one is default and will migrate database automatically upon new server first start up
• manual - will generate update sql query that user manually runs against a database

Mentioned documentation suggest manual update against new database. That is quite hard to achieve as it would require user to create it's own image with appropriate server configuration and injected scripts that would stop server from stopping after generating sql query.

In should be clear from documentation what steps should user take to swich to a new db.