Explain migration histories terminology

[keerlu]

Currently we have this subsection on migration histories.

Would be good to clarify the difference between the migration files and the migration table, and use consistent terminology to refer to each of them.

Also we may want to pull this out into its own separate page as part of a general reorganisation of the information in this page.

Slack discussion here: https://prisma-company.slack.com/archives/CGMCB82N8/p1659705786802819