Add documentation for GHC-83475 (Unused record wildcard)

message-index/messages/GHC-83475/example1/after/Example1.hs

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+{-# OPTIONS_GHC -fwarn-unused-record-wildcards #-}
+{-# LANGUAGE RecordWildCards #-}
+module Example1 where
+
+data Foo = Foo { x :: Int, y :: Int, name :: String }
+
+f :: Foo -> String
+f Foo{} = "Hello"

message-index/messages/GHC-83475/example1/before/Example1.hs

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+{-# OPTIONS_GHC -fwarn-unused-record-wildcards #-}
+{-# LANGUAGE RecordWildCards #-}
+module Example1 where
+
+data Foo = Foo { x :: Int, y :: Int, name :: String }
+
+f :: Foo -> String
+f Foo{..} = "Hello"

message-index/messages/GHC-83475/example1/index.md

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+---
+title: Pattern contains unused record wildcard
+---
+
+None of the fields bound by `Foo{..}` are used in `f`, so the pattern may be safely removed.
+
+## Warning
+
+```
+GHC-83475/example1/before/Example1.hs:7:7: warning: [-Wunused-record-wildcards]
+    No variables bound in the record wildcard match are used
+      Possible fix: omit the ‘..’
+  |
+7 | f Foo{..} = "Hello"
+  |       ^^
+```

message-index/messages/GHC-83475/index.md

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+---
+title: Unused Record Wildcard
+summary: Pattern contains record wildcard that is not used
+severity: warning
+flag: -Wunused-record-wildcards
+introduced: 9.6.1
+---
+
+If `RecordWildCards` is enabled we can automatically bind fields of a record by using patterns like `MyRecord{..}`. Not using any of the record fields that were bound using the record wildcard syntax will result in a warning.