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).
* The command's example uses site aliases but the description of the example refers to directories in the `sites/` folder.

I'm attaching a pull request that should make the documentation more clear.

If possible, I'd like it if these changes made it into the `6.x` branch too. But let's patch `master` first.