Document GHC-15708 (#518)

Author: sergv

message-index/messages/GHC-15708/example1/after/FunDepsDisabled.hs

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+{-# LANGUAGE FunctionalDependencies #-}
+
+module FunDepsDisabled where
+
+class Convert a b | a -> b where
+  convert :: a -> b

message-index/messages/GHC-15708/example1/before/FunDepsDisabled.hs

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+module FunDepsDisabled where
+
+class Convert a b | a -> b where
+  convert :: a -> b

message-index/messages/GHC-15708/example1/index.md

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+---
+title: Missing FunctionalDependencies language extension
+---
+
+Cannot specify functional dependencies for a class - `FunctionalDependencies` language extension is missing.
+
+## Error Message
+
+```
+FunDepsDisabled.hs:1:1: error: [GHC-15708]
+    • Fundeps in class ‘Convert’
+    • In the class declaration for ‘Convert’
+    Suggested fix: Perhaps you intended to use FunctionalDependencies
+  |
+1 | class Convert a b | a -> b where
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
+```

message-index/messages/GHC-15708/index.md

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+---
+title: Missing FunctionalDependencies
+summary: Missing FunctionalDependencies language extension
+severity: error
+introduced: 9.6.1
+---
+
+The [Functional Dependencies extension](https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/functional_dependencies.html#extension-FunctionalDependencies) enables dependencies in multiparameter class declarations.
+
+When the extension is missing, the syntax for specifying dependencies is not recognized.