Documentation fixes

[MeanSquaredError]

This PR documents the changes in the in the ddl2cpp options that were introduced recently. It also fixes a link in the todo document, that points to a discussion about sync support.

[rbock]

Hmm. Changes beyond the .66 release are documented in documented in docs/change_log.md

Maybe that should be organized better?

[MeanSquaredError]

Oh, I see. Maybe I should just drop the PR then?

[rbock]

Please see https://github.com/rbock/sqlpp23/blob/develop/docs/change_log.md#066

Is that less confusing than before?

[MeanSquaredError]

@rbock
I think it is better to keep differences_to_sqlpp11.md as a separate document (like it was before), because IMHO currently more people are interested in migration from sqlpp11 to sqlpp23, than in the changes between different versions of sqlpp23. So it makes sense for the users to have differences_to_sqlpp11.md as a separate file, I think that having the differences placed in the changelog makes it more difficult for the migrating user to find them.

What makes sense is probably expanding the document ddl2cpp.md and describing the different command-line switches of the program.

[rbock]

Thanks for the feedback. I'll undo the changes.

Closing this PR.