Improve the docs around migrations, specifically around streamfield blocks

[Pomax]

Streamfield blocks don't technically count as "model" so all the work technically happens in "PR #1". However, changing a streamfield block can have breaking effects, so it's worth refining the docs a bit to mention what to do when specifically working on streamfield blocks.

[cadecairos]

Relevant post about wagtail data migrations and streamfield: https://www.caktusgroup.com/blog/2019/09/12/wagtail-data-migrations/

[Pomax]

Wagtail somewhat passed this by, so I'm closing this as out of date.