[llvm] [NFC] Add missing bitcode documentation to BitCodeFormat.rst (PR #102555)

[Nikita Popov via llvm-commits]

nikic wrote:

My 2c is that we should probably remove this listing and point people to the source definition. If someone wants to write a 3rd-party bitcode parser (which is the only reason they may need this information), they should be perfectly capable of extracting these numbers from LLVMBitCodes.h...

https://github.com/llvm/llvm-project/pull/102555