Improve sql-sync documentation.

I've always found the sql-sync documentation a bit confusing:

* The command's description, arguments list and examples use different terms for the different arguments.
* The command's description refers to databases you don't specify databases in the arguments, you specify site-aliases or folders.
* The command's arguments list doesn't differentiates between the arguments by name only (the descriptions are the same).

I'm attaching a pull request that should make the documentation more clear.