Update @isdefined to have clearer examples (#36663)

Co-authored-by: Kristoffer Carlsson <kricarl@student.chalmers.se>
Co-authored-by: Matt Bauman <mbauman@juliacomputing.com>

Author: 3 people

base/reflection.jl

@@ -249,6 +249,15 @@ See also [`isdefined`](@ref).
 
 # Examples
 ```jldoctest
+julia> @isdefined newvar
+false
+
+julia> newvar = 1
+1
+
+julia> @isdefined newvar
+true
+
 julia> function f()
            println(@isdefined x)
            x = 3