Return error from at-doc if macroexpand fails.

When `macroexpand` fails due to an undefined macro in the expression
return the resulting error rather than the more generic and misleading
"invalid doc expression..." error message.

Fixes #13905.

(cherry picked from commit c4c38d1)
ref #13908

base/docs/Docs.jl

@@ -541,6 +541,7 @@ function docm(meta, def, define = true)
     isvar(def′)                ? objdoc(meta, def′) :
     isexpr(def′, :tuple)       ? multidoc(meta, def′.args) :
     __doc__!(meta, def′)       ? esc(def′) :
+    isexpr(def′, :error)       ? esc(def′) :
 
     # All other expressions are undocumentable and should be handled on a case-by-case basis
     # with `@__doc__`. Unbound string literals are also undocumentable since they cannot be

test/docs.jl

@@ -471,6 +471,9 @@ end
 @test docstrings_equal(Docs.doc(I13068.A.foo, Tuple{Float64}), doc"foo from B")
 @test Docs.doc(I13068.A.foo, Tuple{Char}) === nothing
 
+# Issue #13905.
+@test macroexpand(:(@doc "" f() = @x)) == Expr(:error, UndefVarError(symbol("@x")))
+
 # Undocumented DataType Summaries.
 
 module Undocumented