As a beginner, I have been looking for quite some time for the possibilities of the different libraries used in the sketches.
More specifically: you use libraries by calling them via a command: ....library.parameter().... e.g.: FastLED.addLeds< >; or FastLED.show(); etc.
This raises the question again and again which possible parameters are also possible and how to use them..?
Is there no documentation about this, or do you have to find out the possibilities (parameters) of each library via examples?? or do you have to investigate the program of this library each time? (e.g.: FastLED.h)..?
Decent libraries come with documentation of their functions and examples of their use
Failing that, you have the source code of the libraries
Some libraries have documentation, typically on their GITHUB repository.
Some libraries are referenced on the Arduino Website, the software serial library for instance.
Some libraries are not formally documented and you need to look at the examples or read the library *.CPP and *.h files.
Many thanks for a reply, I am a hobbyist and have little programming experience. I think Arduino and libraries like FastLED are just right for people like me, they are designed to be as simple and powerful as possible. So I guess I am not allowed to be an experienced programmer. I tried to dig into the header files to learn more, but I didn't understand much of it. I think a very short introduction to the built-in functions would be great, to at least know what is in this great library!
Sorry, my wrong expression, actually I mean: So I think I’m not supposed to be an experienced programmer. 
I think that you picked one of the more (or most) complicated libraries. Although I understand how those addressable LEDs work, I have absolutely no idea how the library works.
Start with some simpler ones like a button library and learn from them.