Added 'IsLibraryLoaded' method to LibraryServices

@Benglin PTAL
I started noticing this when I joined the team, that people leaving the "param" and "returns" tags empty. If they are so straightforward that they don't need explanation, then they should be removed (I have also seen out-of-sync tags). But in this case I do think they deserve their completeness, being a public API: we need to describe a little more about the contract for our callers. For example, what does "library" carry, is that a full file path or just the assembly name? When we write that point, we will be prompted to think about *what if we have two libraries of the same name in different folders, do we consider it loaded?*, then we'll go on to document a little more about the API in its "summary" tag. Also, what's the behavior of this API when "library" is "null" or empty, does it throw an exception? If so what exception?

I hope that makes sense, I think as a team we need to do better in API documentation ;)