[IRBuilder][NFC] Clarify docs on fadd/fmul reductions

This patch aims to clear up any confusion in documentation for the
fadd/fmul reduction creation APIs with regards to the sequential and
unordered variations without changing the APIs themselves.

The scalar accumulator value isn't only used for sequential reduction
intrinsics so the impliciation to the contrary was dropped. Then I
thought it useful to make clear that the API always creates a sequential
reduction. And lastly a note to users on how it is possible to transform
the resulting reduction into an unordered one.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D107753