docs: improve extended examples

Author: MilesCranmer

test/test_expressions.jl

@@ -358,6 +358,10 @@ end
     xs = [Expression(Node{Float64}(; feature=i); operators, variable_names) for i in 1:5]
 
     xs[1] + xs[2]
+    # These have the same type – they simply combine their `AbstractExpressionNode` objects
+    # and ensure the metadata is the same.
+    typeof(xs[1] + xs[2])
+
     #=
     This gives us an easy way to quickly construct expressions with minimal memory overhead,
     and fast evaluation speed:

test/test_structured_expression.jl

@@ -114,7 +114,7 @@ end
     This is a composite `AbstractExpression` object that composes multiple
     expressions during evaluation.
     =#
-    ex = StructuredExpression((; f, g), nt -> nt.f + nt.g)
+    ex = StructuredExpression((; f, g), nt -> nt.f + nt.g; operators, variable_names)
     ex
     @test typeof(ex) <: AbstractExpression{Float64,<:Node{Float64}}  #src
     #=