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

[frasercrmck]

Summary

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.

[craig.topper]

LGTM

llvm/include/llvm/IR/IRBuilder.h

#endif
   /// specified, it will be added to the instruction. Likewise with alias.scope
   /// and noalias tags.
   CallInst *CreateElementUnorderedAtomicMemMove(
       Value *Dst, Align DstAlign, Value *Src, Align SrcAlign, Value *Size,
       uint32_t ElementSize, MDNode *TBAATag = nullptr,
       MDNode *TBAAStructTag = nullptr, MDNode *ScopeTag = nullptr,
       MDNode *NoAliasTag = nullptr);
 
-  /// Create a vector fadd reduction intrinsic of the source vector.
-  /// The first parameter is a scalar accumulator value for ordered reductions.
+  /// Create a sequential vector fadd reduction intrinsic of the source vector.
+  /// The first parameter is a scalar accumulator value. An unordered reduction
+  /// can be created by adding the reassoc fast-math flag to the resulting
+  /// sequential reduction.
   CallInst *CreateFAddReduce(Value *Acc, Value *Src);
 
-  /// Create a vector fmul reduction intrinsic of the source vector.
-  /// The first parameter is a scalar accumulator value for ordered reductions.
+  /// Create a sequential vector fmul reduction intrinsic of the source vector.
+  /// The first parameter is a scalar accumulator value. An unordered reduction
+  /// can be created by adding the reassoc fast-math flag to the resulting
+  /// sequential reduction.
   CallInst *CreateFMulReduce(Value *Acc, Value *Src);
 
   /// Create a vector int add reduction intrinsic of the source vector.
   CallInst *CreateAddReduce(Value *Src);
 
   /// Create a vector int mul reduction intrinsic of the source vector.
   CallInst *CreateMulReduce(Value *Src);