add tests for undocumented symbols (#52723)

Following #52413 by @jariji and the discussion in #51174, this adds
tests to ensure that there are no undocumented *(= lacking docstrings)*
public symbols in any documented module.

Many of the tests are broken — we have a lot of undocumented exports. In
such cases I added both a `@test_broken` and a `@test` to ensure that
there are no additional regressions added in the future.

~~(This PR may have to be updated when
JuliaLang/julia#52413 (comment) is
fixed, i.e. when ~~#52727~~ #52743 is merged.)~~ Has been updated.

---------

Co-authored-by: Lilith Orion Hafner <lilithhafner@gmail.com>

test/runtests.jl

@@ -324,3 +324,7 @@ end
 @test SharedMatrix([0.1 0.2; 0.3 0.4]) == [0.1 0.2; 0.3 0.4]
 @test_throws MethodError SharedVector(rand(4,4))
 @test_throws MethodError SharedMatrix(rand(4))
+
+@testset "Docstrings" begin
+    @test isempty(Docs.undocumented_names(SharedArrays))
+end