Docs: consider mentioning :identifiers and :as-rows? options

The official guide could be enhanced with some mentions of `:identifiers` and `:as-rows?`.

Specifically, I recall I spent some time before I figured out how to "kebab-case-keyword"-ize the fetched results as there's nothing about it directly in `jdbc.core/fetch` docstring (they are mentioned in [jdbc.resultset](https://github.com/funcool/clojure.jdbc/blob/775cb9ecc68e6f1898943044ecc226920244d057/src/jdbc/resultset.clj#L32) docs):

```clj
(ns foo.bar
(:require [camel-snake-kebab.core :as csk]
[jdbc.core :as jdbc]))
...
(jdbc/fetch conn sql {:identifiers csk/->kebab-case-keyword})
```