Add news and compat for mul!(C, A, B, α, β) (#32900)

NEWS.md

@@ -74,6 +74,7 @@ Standard library changes
 * The BLAS submodule no longer exports `dot`, which conflicts with that in LinearAlgebra ([#31838]).
 * `diagm` and `spdiagm` now accept optional `m,n` initial arguments to specify a size ([#31654]).
 * `Hessenberg` factorizations `H` now support efficient shifted solves `(H+µI) \ b` and determinants, and use a specialized tridiagonal factorization for Hermitian matrices. There is also a new `UpperHessenberg` matrix type ([#31853]).
+* Five-argument `mul!(C, A, B, α, β)` now implements inplace multiplication fused with addition _C = A B α + C β_ ([#23919]).
 
 #### SparseArrays
 

stdlib/LinearAlgebra/src/matmul.jl

@@ -202,6 +202,9 @@ Combined inplace matrix-matrix or matrix-vector multiply-add ``A B α + C β``.
 The result is stored in `C` by overwriting it.  Note that `C` must not be
 aliased with either `A` or `B`.
 
+!!! compat "Julia 1.3"
+    Five-argument `mul!` requires at least Julia 1.3.
+
 # Examples
 ```jldoctest
 julia> A=[1.0 2.0; 3.0 4.0]; B=[1.0 1.0; 1.0 1.0]; C=[1.0 2.0; 3.0 4.0];