[cleanup] Modernize doxygen comments for the BumpPtrAllocator and
rewrite some of them to be more clear.

The terminology being used in our allocators is making me really sad. We
call things slab allocators that aren't at all slab allocators. It is
quite confusing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204907 91177308-0d34-0410-b5e6-96231b3b80d8