Docs: describe behaviour of `FromStrAsJson` in case type deserialises from a JSON string (opposed to an object or array, for example)

[jofas]

The current behaviour might be confusing when the type deserialises from a JSON string (see #13). Describing that the &str passed to FromStr::from_str is treated as a container containing the JSON string (i.e. the string slice must start and end with " so the content is a valid JSON string) and not the JSON string itself hopefully mitigates further confusion. The example I gave here could be enhanced and used as an example for the docs.

Also, the use of the expression "json string" is imprecise in the current docs. This should be amended, too, to avoid even more confusion.