[docs] Update DominatorTree docs to clarify expectations around unreachable blocks

Note: I'm am not trying to describe what "should be"; I'm only describing what is true today.

This came out of my recent question to llvm-dev titled: When can the dominator tree not contain a node for a basic block?

Differential Revision: http://reviews.llvm.org/D13078

llvm-svn: 248417