[llvm] docs: Adjust HowToReleaseLLVM to include man page signing (PR #210092)

[Konrad Kleine via llvm-commits]

kwk wrote:

> > If possible, a release manager can apply the changes to the already released [LLVM-23.1.0-rc1](https://github.com/llvm/llvm-project/releases#release-llvmorg-23.1.0-rc1) release?
> 
> I don't think rc1 has been released yet, but @dyung is aware this also needs to be signed now so it should be taken care of when release happens

Sorry, I manually constructed the URL and it led to the 22 release without me noticing.

Great that that @dyung is aware.

https://github.com/llvm/llvm-project/pull/210092