text: Title: Make JSONHandler customization docs clearer
  Body: As pointed out by @Stargateur in https://github.com/falconry/falcon/issues/1906#issuecomment-817374057, our [`JSONHandler`](https://falcon.readthedocs.io/en/stable/api/media.html#falcon.media.JSONHandler) customization docs could be made clearer by separately illustrating different (albeit closely related) concepts:
  * Use a custom JSON library (such as the exemplified `rapidjson`). Customize parameters.
  * Use the stdlib's `json` module, just provide custom serialization or deserialization parameters. Also link to the ["Prettifying JSON Responses" recipe](https://falcon.readthedocs.io/en/stable/user/recipes/pretty-json.html), which illustrates customization of `dumps` parameters.
  * Add a sentence or two about replacing the default JSON handlers, not just toss in a code snippet as it is at the time of writing this. Also link to [Replacing the Default Handlers](https://falcon.readthedocs.io/en/stable/api/media.html#custom-media-handlers) from that explanation.