Clarify what autogenerate compares

Clarify what autogenerate compares

### Description
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I was struggling with empty upgrade/downgrade functions because both my database and my ORM were up to date. I wrongly assumed the comparison was against the previous database scheme as per the prior revision script(s).

### Checklist
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This pull request is:

- [x] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #1570
Pull-request: https://github.com/sqlalchemy/alembic/pull/1570
Pull-request-sha: dec968b136a813d27e1d2bca04574a2399343a1a

Change-Id: I04dadc3c3d26ad000aca499208d8024cd63eec40