Add docs for data migrations

Add docs for data migrations

### Description
I added a small docs regarding data migrations, based on #972.

### Checklist
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This pull request is:

- [X] A documentation / typographical error fix
	- Good to go, no issue or tests are needed

**Have a nice day!**

Closes: #1040
Pull-request: https://github.com/sqlalchemy/alembic/pull/1040
Pull-request-sha: c985e0d8096e750df35aa07dde71f432a57656b6

Change-Id: Icf6fdb1089eb7c386dad4401b99caaddc352d92f