Need better documentation of selection behavior

jirikadlec2[CodePlex]
Requested in issue:
http://hydrodesktop.codeplex.com/workitem/8555
we need better documentation for selection in the help files and quick start guide.

Technical description of selection behavior is:

n current version of HD the selection works like this:

If the 'Map Layers' item is highlighted in the legend (All Layers Selected), then selection can select features from all layers.
If one of the layers is highlighted in the legend (for example, the U.S States is highlighted and status bar shows 'Layer: U.S States', then selection can only select features from the selected layer.
If one of the categories is highlighted, for example 'Alabama' in U.S states, then selection doesn't work