nixos/nginx: improve documentation for `config`

Unfortunately, I had a use-case where `services.nginx.config` was
necessary quite recently. While working on that config I had to look up
the module's code to understand which options can be used and which
don't.

To slightly improve the situation, I changed the documentation like
this:

* Added `types.str` as type since `config` is not mergeable on purpose.
  It must be a string as it's rendered verbatim into `nginx.conf` and if
  the type is `unspecified`, it can be confused with RFC42-like options.

* Mention which config options that don't generate config in
  `nginx.conf` are NOT mutually exclusive.

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