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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > Most of these could use more elaboration, especially earlyclobber and renamable
> 
> These are just copy pasted from the enum value comments

Where I usually go to refer to them, and still don't understand what renamable actually does 

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