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 <[email protected]>

test/runtests.jl

@@ -1,5 +1,12 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
+using Test, LinearAlgebra
 
 for file in readlines(joinpath(@__DIR__, "testgroups"))
     include(file * ".jl")
 end
+
+@testset "Docstrings" begin
+    undoc = Docs.undocumented_names(LinearAlgebra)
+    @test_broken isempty(undoc)
+    @test undoc == [:ColumnNorm, :LAPACKException, :NoPivot, :RankDeficientException, :RowMaximum, :RowNonZero, :copy_transpose!]
+end