Update database documents on new text column enforcement and cop rules

With the new rules implemented in the following MRs:

• Enforce text columns with a limit over strings on migrations - !29304 (merged)
• Add migration helpers for managing check constraints and limits on text columns - !29181 (merged)

we need to update https://docs.gitlab.com/ee/development/README.html#database-guides to reflect these new guidelines.

The description from !29304 (merged) is a good starting point for this documentation.