Add existing 3-arg * docstring to the manual (#53288)

PR #37898 added methods to `*` for chained matrix multiplication. They
have a descriptive docstring but I don't think this is mentioned in the
manual.

Author: mcabbott

stdlib/LinearAlgebra/docs/src/index.md

@@ -412,6 +412,7 @@ Other sparse solvers are available as Julia packages.
 
 ```@docs
 Base.:*(::AbstractMatrix, ::AbstractMatrix)
+Base.:*(::AbstractMatrix, ::AbstractMatrix, ::AbstractVector)
 Base.:\(::AbstractMatrix, ::AbstractVecOrMat)
 Base.:/(::AbstractVecOrMat, ::AbstractVecOrMat)
 LinearAlgebra.SingularException