C(5.5) Migrations: Improve explanation

[ricayanzon]

I think the concept of migrations can be misunderstood by junior devs, i.e. they could wonder why you don't just have a drizzle function to alter the table, similar to how it was done in the previous examples in the database directly using SQL. More than just helping you to update a table, migrations are beneficial because they allow version control (e.g. for rollbacks), consistency/reproducibility between your current state and that of others, and are a form of documentation.
Also, for readers that try out stuff while reading, the table that is created in that example should already exist. I'd be a small change but better imo if a new table name is used.