Document that XGhostState is only defined for LLVM at the moment

I've included a pointer to #1929 as well to indicate our intention to support
ghost state on other language backends.

src/SAWScript/Crucible/Common/Setup/Value.hs

@@ -175,11 +175,10 @@ deriving instance (SetupValueHas Show ext) => Show (SetupValue ext)
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 -- ** Ghost state
 
--- TODO: This is language-independent, it should be always-true rather than a
--- toggle.
-
--- TODO: documentation
-
+-- | This extension field controls whether ghost state is enabled for a
+-- particular language backend. At the moment, ghost state is only enabled for
+-- the LLVM backend, but we want to expand this to cover other language backends
+-- in the future. See <https://github.com/GaloisInc/saw-script/issues/1929>.
 type family XGhostState ext
 
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