Docs: add examples to istril/istriu docstrings (#49853)

Also, simplify some of the examples to focus on the structure of the
matrices

stdlib/LinearAlgebra/src/generic.jl

@@ -1284,16 +1284,17 @@ false
 julia> istriu(a, -1)
 true
 
-julia> b = [1 im; 0 -1]
-2×2 Matrix{Complex{Int64}}:
- 1+0im   0+1im
- 0+0im  -1+0im
-
-julia> istriu(b)
-true
+julia> c = [1 1 1; 1 1 1; 0 1 1]
+3×3 Matrix{Int64}:
+ 1  1  1
+ 1  1  1
+ 0  1  1
 
-julia> istriu(b, 1)
+julia> istriu(c)
 false
+
+julia> istriu(c, -1)
+true
 ```
 """
 function istriu(A::AbstractMatrix, k::Integer = 0)
@@ -1328,16 +1329,17 @@ false
 julia> istril(a, 1)
 true
 
-julia> b = [1 0; -im -1]
-2×2 Matrix{Complex{Int64}}:
- 1+0im   0+0im
- 0-1im  -1+0im
-
-julia> istril(b)
-true
+julia> c = [1 1 0; 1 1 1; 1 1 1]
+3×3 Matrix{Int64}:
+ 1  1  0
+ 1  1  1
+ 1  1  1
 
-julia> istril(b, -1)
+julia> istril(c)
 false
+
+julia> istril(c, 1)
+true
 ```
 """
 function istril(A::AbstractMatrix, k::Integer = 0)