[llvm] [LV][doc] Update and extend the docs on floating-point reduction vectorization (PR #172809)

[Tibor Győri via llvm-commits]

TiborGY wrote:

Since LLVMbot does not seem to trigger on prue docs updates, I am tagging @llvm/pr-subscribers-vectorizers manually.

https://github.com/llvm/llvm-project/pull/172809