Fixes regarding densities in "Providing Resources" documentation page.

The ratio between densities is not constant.

The Exception for dpi qualifier in point 4 of the algorithm should be detailed.

I understand that if my config is ldpi-trackball-v5
and I have the choice between mdpi and hdpi-trackball-v5, mdpi will be chosen.

Change-Id: Icf7494c23f7e3af7d3c6aab770d0ba3c1fea2ebf