doc: elabissue for variable universe bug

tests/elabissues/variable_universe_bug.lean

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+/-
+Courtesy of @rwbarton.
+
+This is just a bug report, but since we are going to soon rewrite the entire module in Lean4,
+we don't want to bother fixing the C++ bug, and we don't want to add a failing test either.
+
+The issue is that collecting implicit locals does not collect additional universe parameters
+the locals depend on.
+-/
+universes v u
+class Category (C : Type u) :=
+(Hom : ∀ (X Y : C), Type v)
+
+variables {C : Type u} [Category.{v, u} C]
+
+def End (X : C) := Category.Hom X X -- invalid reference to undefined universe level parameter 'v'