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.

Cherry-picked from master CL 59188.

Change-Id: Ibe412a82845527ebdb104605c99c205ca52ab97c