Documentation Update re executeUpdateWithFormat and executeQueryWithFormat

In response to a [discussion on Stack Overflow](http://stackoverflow.com/questions/18085019/ios-sqlite-insert-query-not-working), I thought a clarification on the issues associated with the `executeUpdateWithFormat` and `executeQueryWithFormat` was warranted. I (a) expanded the rationale regarding the warning (as opposed to just warning them); and (b) clarified the discussion on the parameters to the standard `executeUpdate` and `executeQuery` methods.

I could, theoretically, add this discussion to the `FMDatabaseAdditions` class that also take variable arguments, but did not do that, as I hope we've addressed the issue sufficiently here.

I also, while doing this, noticed a minor spacing problem in `FMDatabase.m` and also fixed the reference to `UTF8StringForColumnName` and `objectForColumnName` in the README (but did _not_ change any method names in code, per our previous discussion).

I'll issue an associated, separate gh-pages pull request for the resulting documentation changes that stem from updating the `.h` files here.