doc: document private keyword elaboration issues

tests/elabissues/private.lean

@@ -0,0 +1,35 @@
+-- Issue 1
+def foo := 10
+
+def f (x : Nat) := x + x
+
+namespace Bla
+
+private def foo := "hello"
+
+#check foo -- error: ambiguous overload. It should resolve to `Bla.foo`
+
+end Bla
+
+-- Issue 2
+namespace Boo
+
+def boo := 100
+
+namespace Bla
+
+private def boo := "hello"
+
+#check boo -- resolving to `Boo.boo` instead of `Boo.Bla.boo`
+
+#check boo ++ "world" -- error since it is resolving to `Boo.Bla.boo`
+
+end Bla
+
+end Boo
+
+-- Issue 3
+private def Nat.mul10 (x : Nat) := x * 10
+def x := 10
+
+#eval x.mul10 -- error, field notation does not work for private definitions