Improve documentation of withBind, plus a couple tweaks

This adds documentation to withBind explaining the rationale behind resetting the statement after use, instead of before.  Namely, it avoids a pitfall involving implicit transactions.

The other commits are minor tweaks you may or may not want:

 * a60998b: Use `finally` instead of `bracket` in `withBind`.  This means the `bind` step is no longer under an async exception mask.  `bind` is not "acquiring" the same resource that `reset` is relinquishing, as `reset` does not reset bound parameters.

 * f2e9f59: In `convertRow`, use `throwIO` in the first case, like the other cases do.

 * Don't use named imports for `Control.Exception`.