[llvm] [NFC] add explanation to register flags doc (PR #91803)

[Matt Arsenault via llvm-commits]

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@@ -540,6 +540,9 @@ Register Flags
 The table below shows all of the possible register flags along with the
 corresponding internal ``llvm::RegState`` representation:
 
+..
+   Keep this in sync with MachineInstrBuilder.h
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arsenm wrote:

Does this appear in the generated output? 

https://github.com/llvm/llvm-project/pull/91803