For example in %Library.File Class Documentation,  FiieSet query is listed as: 
"Selects Name As %String, Type As %String, Size As %BigInt, DateCreated As %TimeStamp, DateModified As %TimeStamp, ItemName As %Stringof ..."

How to obtain a "real" syntax?

Thank you.

Please, provide Code example to build a %Library.DynamicObject or a %Library.DynamicArray from Resultset followed by converting to JSON