Document GHC-08838 (#543)

* Document GHC-08838

* Update message-index/messages/GHC-08838/index.md

---------

Co-authored-by: David Binder <[email protected]>

message-index/messages/GHC-08838/WrongDoBind/after/WrongDoBind.hs

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+module WrongDoBind where
+
+doubleReturn :: forall m. Monad m => m Int
+doubleReturn = return 10

message-index/messages/GHC-08838/WrongDoBind/before/WrongDoBind.hs

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+module WrongDoBind where
+
+doubleReturn :: forall m. Monad m => m Int
+doubleReturn = do
+  return (return 10 :: m Int)
+  return 10

message-index/messages/GHC-08838/WrongDoBind/index.md

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+
+---
+title: Double return
+---
+
+In this example, there is a nested return whose result is not bound to a variable. Therefore the computation `return 10` is not used and can be removed.
+
+```
+messages/GHC-08838/WrongDoBind/before/WrongDoBind.hs:5:4: warning: [GHC-08838] [-Wwrong-do-bind]
+    A do-notation statement discarded a result of type ‘m Int’
+    Suggested fix:
+      Suppress this warning by saying ‘_ <- return (return 10 :: m Int)’
+  |
+5 |    return (return 10 :: m Int)
+  |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+```

message-index/messages/GHC-08838/index.md

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+---
+title: Wrong do bind
+summary: Returning a monadic computation in a do block and not binding it will not run it
+severity: warning
+introduced: 9.6.1
+---
+When using a monadic computation in a do block one should either run it 
+directly or bind the result of the computation to a variable. If neither 
+is done the monadic computation is never run and can be removed.