Document HiddenConstructors (documentationjs#354)

* Document `HiddenConstructors`

Author: kl0tl

errors/HiddenConstructors.md

@@ -0,0 +1,42 @@
+# `HiddenConstructors` Warning
+
+## Example
+
+```purescript
+module ShortFailingExample (N) where
+
+import Data.Newtype (class Newtype)
+import Data.Generic.Rep (class Generic)
+
+newtype N a = MkN a
+
+derive instance newtypeN :: Newtype (N a) _
+derive instance genericN :: Generic (N a) _
+```
+
+## Cause
+
+Instances of `Newtype` and `Generic` allow to match values of their type with `unwrap` and `from`, and to construct them with `wrap` or `to` hence making the constructors public.
+
+## Fix
+
+- Export the constructors:
+
+```diff
+-module ShortFailingExample (N) where
++module ShortFailingExample (N(..)) where
+```
+
+- or remove the instances:
+
+```diff
+ module ShortFailingExample (N) where
+
+-import Data.Newtype (class Newtype)
+-import Data.Generic.Rep (class Generic)
+
+ newtype N a = MkN a
+
+-derive instance newtypeN :: Newtype (N a) _
+-derive instance genericN :: Generic (N a) _
+```