Improve comments in pravega-samples code

[RaulGracia]

Problem description
Sample applications comments should not be as technical and concise as in the main project's code; that is, adding a couple of comment lines in a class/method explaining an important concept (i.e., StreamCut, ReaderGroup) before describing what the code does may be a good idea.

Problem location
Samples code (comments).

Suggestions for an improvement
Review if existing samples lack from comments and improve them.