Add section to guide to describe the difference between session relevance and client relevance

[gearoidibm]

This is something that beginners find difficult to deal with. Add a section that calls out the difference and also gives intro to the different types of objects for common objects like bes computer, bes fixlet and so on.

[gabrisan]

Hi, I just saw this request, I added this description a few months ago: https://developer.bigfix.com/relevance/. I was also thinking with Aaron, how we could be split the two types of relevance languages to avoid confusion.What do you think about it? Ciao Gabriella

[gearoidibm]

@gabrisan's update works. Closing.