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

[Matt Arsenault via llvm-commits]

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@@ -545,36 +545,47 @@ corresponding internal ``llvm::RegState`` representation:
 
    * - Flag
      - Internal Value
+     - Meaning
 
    * - ``implicit``
      - ``RegState::Implicit``
+     - Not emitted register (e.g. carry, or temporary result).
 
    * - ``implicit-def``
      - ``RegState::ImplicitDefine``
+     - ``implicit`` and ``def``
 
    * - ``def``
      - ``RegState::Define``
+     - Register definition.
 
    * - ``dead``
      - ``RegState::Dead``
+     - Unused definition.
 
    * - ``killed``
      - ``RegState::Kill``
+     - The last use of a register.
 
    * - ``undef``
      - ``RegState::Undef``
+     - Value of the register doesn't matter.
 
    * - ``internal``
      - ``RegState::InternalRead``
+     - Register reads a value that is defined inside the same instruction or bundle.
 
    * - ``early-clobber``
      - ``RegState::EarlyClobber``
+     - Register definition happens before uses.
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arsenm wrote:

This one probably needs more elaboration 

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