Thanks for the great documentation.
We believe it's missing a very important fact about large chunks, which could be cause by incorrect selection of a shard key. Unsplittable large chunks will not be moved by the balancer. Thus, making them static to the shard they've been created on and never be able to move them, in case there's a need for it.
In "Scaling MongoDB" Kristina Chodorow writes "If a chunk gets too big, MongoDB will refuse to move it". This comment is spot on and should be in the official documentation as it affects the shard key decision.