Update migration style guide with advice for column types

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The following discussion from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2807#note_39268328 should be addressed:

• @nick.thomas started a discussion:

  Interesting advice! We should probably try to get some of it into doc/development/migration_style_guide.md

We should update the migration style guide with some advice related to database types.