Add an IndexStyle example to the diagind docstring (#53757)

Also, simplifies the docstring by splitting the usage into two lines.

(cherry picked from commit 0b1587c)

stdlib/LinearAlgebra/src/dense.jl

@@ -216,13 +216,16 @@ function diagind(::IndexCartesian, m::Integer, n::Integer, k::Integer=0)
 end
 
 """
-    diagind(M::AbstractMatrix, [k::Integer=0,] indstyle::IndexStyle = IndexLinear())
+    diagind(M::AbstractMatrix, k::Integer = 0, indstyle::IndexStyle = IndexLinear())
+    diagind(M::AbstractMatrix, indstyle::IndexStyle = IndexLinear())
 
 An `AbstractRange` giving the indices of the `k`th diagonal of the matrix `M`.
 Optionally, an index style may be specified which determines the type of the range returned.
 If `indstyle isa IndexLinear` (default), this returns an `AbstractRange{Integer}`.
 On the other hand, if `indstyle isa IndexCartesian`, this returns an `AbstractRange{CartesianIndex{2}}`.
 
+If `k` is not provided, it is assumed to be `0` (corresponding to the main diagonal).
+
 See also: [`diag`](@ref), [`diagm`](@ref), [`Diagonal`](@ref).
 
 # Examples
@@ -233,9 +236,15 @@ julia> A = [1 2 3; 4 5 6; 7 8 9]
  4  5  6
  7  8  9
 
-julia> diagind(A,-1)
+julia> diagind(A, -1)
 2:4:6
+
+julia> diagind(A, IndexCartesian())
+StepRangeLen(CartesianIndex(1, 1), CartesianIndex(1, 1), 3)
 ```
+
+!!! compat "Julia 1.11"
+     Specifying an `IndexStyle` requires at least Julia 1.11.
 """
 function diagind(A::AbstractMatrix, k::Integer=0, indexstyle::IndexStyle = IndexLinear())
     require_one_based_indexing(A)