GHC-81995: add error message and example using sequence

message-index/messages/GHC-81995/index.md

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+---
+title: Unused "do" bind
+summary: A return value of a monadic action is ignored
+severity: warning
+flag: -Wunused-do-bind
+introduced: 9.6.1.
+---
+
+Many monadic actions perform side effects before returning a value. For example,
+`sequence :: (Traversable t, Monad m) => t (m a) -> m (t a)` performs all
+actions in a traversable container and returns the container with the return
+values. Sometimes, we are only interested in the side effect of a monadic
+actions, and not the return value. In these cases, we should use appropriate
+functions indicating that we purposefully ignore the return value.

message-index/messages/GHC-81995/sequence/after/GetLine.hs

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+module GetLine where
+
+main :: IO ()
+main = do
+  sequence_ [putStrLn "First line", putStrLn "Second line"]
+  putStrLn "Done."

message-index/messages/GHC-81995/sequence/before/GetLine.hs

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+module GetLine where
+
+main :: IO ()
+main = do
+  sequence [putStrLn "First line", putStrLn "Second line"]
+  putStrLn "Done."

message-index/messages/GHC-81995/sequence/index.md

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+---
+title: Fix warning about unused "do" bind using `sequence_`
+---