Database sizing instruction for upgrade

When running a SonarQube migration, we sometimes copy entire tables, to be faster.

The downside is that the migration use a lot of space, and users may run into errors with a full filesystem (The transaction log for database 'sonarqube' is full due to 'ACTIVE_TRANSACTION'.)

We need to raise awareness about this, to warn users that they need to extends their storage capacity before starting a huge migration.

As a rule of thumb, the database storage should be below 50% usage.