Fix isBaseLayer documentation

Ensure the documentation matches the code with regards to the default value, and make the explanation slightly clearer as to why it defaults to false.
Backbuffer needs z-index

Since we don't rely on the DOM order for layers, we also cannot rely on it
for backbuffers. This change simply adds a sensible z-index to the
backbuffer container.
Merge pull request #921 from gravitystorm/patch-5

Fix UTFGrid isBaseLayer documentation
Making backbuffer order configurable

In OpenLayers 2.12, the backbuffer for each layer was placed right below
the layer itself. With animated zooming, that was changed so backbuffers
are all placed below the base layer. As it turned out, this is only good
for certain kinds of layers (e.g. street labels), so the default now is
to place the backbuffer right below its layer again. But the
transitionEffect option can now have a new value, 'map-resize', which will
cause the backbuffer to be placed below the base layer.
Typo in API docs; use "map-resize" option in example
Merge pull request #922 from ahocevar/backbuffer-zindex

Configurable backbuffer order. r=@bartvde
Merge branch 'master' of github.com:openlayers/openlayers into dimNGfresh