Improve documentation for animation-related stuff

The animation-related code is missing sufficient detail to make it
usable without digging in to the code.  This adds some additional
detail to the ol.animation namespace, the related options, and makes
note of the existance of ol.animation in the documentation of
ol.Map.addPreRenderFunction(s).
remove trailing whitespace to make the linter happy.
Merge pull request #950 from pagameba/doc-animations

Improve documentation for animation-related stuff.