More clear description of sendJSON() method

[mildabre]

I propose describe the function of sendJSON() method more clear, in current version is written: "Příklad action-metody, která odešle data ve formátu JSON", but the data are sent not by action method, but by sendJSON() method.

Also it is important add info, that the data are both encoded into JSON and send as http response, to avoid confusing that you should pass into the method data already encoded in JSON - small difference in meaning but important!