Add documentation for GHC-85401 (missing export lists) (#520)

Author: kaitanie

message-index/messages/GHC-85401/example1/after/Example1.hs

@@ -0,0 +1,5 @@
+{-# OPTIONS_GHC -Wmissing-export-lists #-}
+module Example1 (hello) where
+
+hello :: String -> String
+hello s = "Hello " <> s

message-index/messages/GHC-85401/example1/before/Example1.hs

@@ -0,0 +1,5 @@
+{-# OPTIONS_GHC -Wmissing-export-lists #-}
+module Example1 where
+
+hello :: String -> String
+hello s = "Hello " <> s

message-index/messages/GHC-85401/example1/index.md

@@ -0,0 +1,13 @@
+---
+title: Export list is missing in module declaration
+---
+
+## Warning
+
+```haskell
+Example1.hs:1:1: warning: [-Wmissing-export-lists]
+    The export item ‘module Example1’ is missing an export list
+  |
+1 | {-# OPTIONS_GHC -Wmissing-export-lists #-}
+  | ^
+```

message-index/messages/GHC-85401/index.md

@@ -0,0 +1,9 @@
+---
+title: Missing Export List
+summary: The module export list is missing
+severity: warning
+flag: -Wmissing-export-lists
+introduced: 9.6.1
+---
+
+If enabled by `-Wmissing-export-lists` or `-Weverything`, GHC issues a warning when the `module` declaration does not contain an export list. When an export list is missing, all definitions in the module are exported.