[ARM64-BE] Add sphinx documentation for the ARM64 NEON implementation.

There are some interesting decisions based on non-obvious rationale in
the ARM64-BE NEON implementation - decent documentation is definitely required.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208577 91177308-0d34-0410-b5e6-96231b3b80d8