Documentation fixes

Carrying on with the [work done on the ORM](https://github.com/doctrine/doctrine2/pull/528), here are my documentation fixes for the DBAL library:

* Missing docblocks
* Missing / incorrect `@param` / `@return` / `@throws` annotations
* Missing newlines between annotations
* Incorrect vertical alignment of `@param` annotations

In addition:

* Removal of `return` statements on `void` methods
* Cleanup of unused private / local variables