doc: elabissue for bind with existential types

tests/elabissues/bind_with_existential_types.lean

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+/-
+Problem: existential types do not play well with monads.
+
+```
+class HasBind (m : Type u → Type v) :=
+(bind : ∀ {α β : Type u}, m α → (α → m β) → m β)
+```
+
+Even if we fix a universe-polymorphic monad M.{u v} : Type u -> Type v,
+the bind operator forces the universes of the two applications of M to be the same.
+So, we can not naively write:
+-/
+
+universes u v
+axiom Foo : Type u → Type v
+@[instance] axiom fooMonad : Monad Foo
+
+noncomputable def doesNotWork : Foo Unit := do
+x ← pure (5 : Nat);
+y ← pure $ x + 3;
+Z ← pure (Nat : Type);
+pure ()
+
+/- It yields the following error:
+
+<<
+bind_with_existential_types.lean:18:18: error: type mismatch at application
+  pure Nat
+term
+  Nat
+has type
+  Type : Type 1
+but is expected to have type
+  ?m_1 : Type
+>>
+-/
+
+/- We can make it work by adding a bunch of ups and downs: -/
+
+noncomputable def moreTedious : Foo Unit := ULift.down <$> do
+  x ← pure $ ULift.up (5 : Nat);
+  y ← pure $ ULift.up (ULift.down x + 5);
+  z ← pure (Nat : Type);
+  pure $ ULift.up ()
+
+/-
+Without special elaborator support to insert all the ups and downs,
+it would be highly annoying to write State monad code that uses an existential type,
+e.g. a structure that simulates a Coq/ML module by bundling a type and a map API on the type.
+
+Note that one cannot vary universes at all in generic monadic combinators,
+no matter how many lifts and downlifts you are willing to perform,
+since only definitions can be universe polymorphic.
+If a definition takes a monad as an argument, its universe is already fixed.
+-/