[NFC][LLVM][LangRef] Improve documentation for partial.reduce.add. (#126728)

Author: paulwalker-arm

llvm/docs/LangRef.rst

@@ -20238,18 +20238,31 @@ Overview:
 """""""""
 
 The '``llvm.vector.experimental.partial.reduce.add.*``' intrinsics reduce the
-concatenation of the two vector operands down to the number of elements dictated
-by the result type. The result type is a vector type that matches the type of the
-first operand vector.
+concatenation of the two vector arguments down to the number of elements of the
+result vector type.
 
 Arguments:
 """"""""""
 
-Both arguments must be vectors of matching element types. The first argument type must
-match the result type, while the second argument type must have a vector length that is a
-positive integer multiple of the first vector/result type. The arguments must be either be
-both fixed or both scalable vectors.
+The first argument is an integer vector with the same type as the result.
 
+The second argument is a vector with a length that is a known integer multiple
+of the result's type, while maintaining the same element type.
+
+Semantics:
+""""""""""
+
+Other than the reduction operator (e.g. add) the way in which the concatinated
+arguments is reduced is entirely unspecified. By their nature these intrinsics
+are not expected to be useful in isolation but instead implement the first phase
+of an overall reduction operation.
+
+The typical use case is loop vectorization where reductions are split into an
+in-loop phase, where maintaining an unordered vector result is important for
+performance, and an out-of-loop phase to calculate the final scalar result.
+
+By not introducing any new ordering constraints these intrinsics maximize the
+abilitity to utilise a target's accumulation instructions.
 
 '``llvm.experimental.vector.histogram.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^