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

[Stephen Tozer via llvm-commits]

SLTozer 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...

+1, I noticed when adding the new bitcode value that some of the existing bitcode attributes weren't included in the list as well. It seems like duplication that will inevitably trip up someone when we fail to update it. If there was a useful way to auto-generate docs from the enum that would be useful too, but pointing to the source SGTM. That said, this patch LGTM since it's trivially fixing an existing problem.

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